"Tube-style" map of Roman roads

[Swampster]

I am not sure Londinium ever was the capital during Roman times: that honour generally fell to Camulodunum (Camelot/Colchester), which has been a military town more or less since its inception.

Originally rather than generally?

Perhaps.  There is no clear evidence of Londinium ever being the formal capital, or at least none of which I am aware.  There are plenty of hints that it occupied a position of significance in the administration of Britannia, but we may note that when Constantius Chlorus died and his son Constantine was proclaimed emperor, it was at Eboracum (York).  Camulodunum however contains the largest theatre and only known chariot circus in Britannia, which suggests a degree of continuing importance.

London probably had a circus as yet not found. Camulodunum's amphitheatre may have been found but not yet proven - the largest yet is in Chester. I wonder if London had more than one amphitheatre - the one under the Guildhall is within the city walls which is fairly unusual.
Camulodunum has the largest theatre _found_ in Britain - apparently there are only five and two of these are by Colchester.

York had its benefits for directing military operations - Septimius Severus also operated from there. C. Chlorus was overwintering there, probably intending to return further north but stayed in York once he was dying.

[Swampster]

I suspect the port of London is the key thing. Sure, there was cross-channel traffic pre-invasion, but by using London the Romans could get sea traffic not just further north but also some way into the country and closer to the initial areas of operations. The legions then fan out from there, building the roads as they go.
The area around the port becomes more developed as it has the transport infrastructure - I suspect there was a good deal of riverine traffic too. After all, there are only a small number of rivers going into the heart of England and the Thames is the most convenient for Continental traffic.

In other words, the roads didn't lead to or from London. London grew because of the roads.

As trade declined with the retreat of empire, London also declined. As trade started to pick up, London became resettled, then suffered under Viking attack.
Winchester and the other A/S capitals were more politically important being both more secure (in theory) and more central. Of the four main kingdoms, the two inland capitals are both on Roman roads. Tamworth is also on the Tame, a major tributary of the Trent, allowing riverine trade within and beyond the kingdom. Had Ellendun gone the other way, perhaps Tamworth would have developed as capital of a united England. (Mercian hegemony would, of course, have made the country strong enough to drive out the Vikings before they could get established. All hail Mercia!)

[Imperial Dave]

so that combination of major waterway and early Roman road access gave London an edge in terms of its location although its preeminence wasnt possibly realised until late on....

[Erpingham]

so that combination of major waterway and early Roman road access gave London an edge in terms of its location although its preeminence wasnt possibly realised until late on....

The power of economic geography strikes again.

[Swampster]

I wonder how much of the size of London is down to its economic importance and how much due to political. The economic thing still applies now, even with the crazy building costs, with companies setting up there because that is where so many other businesses are. Is it a case of the economic importance creating the political importance which enhances the economic importance and so on?
Does the same thing apply to Paris? It has some advantages of the river, and it is also at centre of a road network. It grew large pretty early on but is that because Clovis had chosen it as capital or was it a geographical thing?
London and Paris are certainly far larger than the other OECD 'Large Urban Zones' both in their own countries and the other European LUZs. Italy on the other hand has three very equally sized LUZs around Milan, Rome and Naples. I suppose Rome has the benefit of being central but not much else. Germany also has little difference between Berlin and the Ruhr, with quite a few LUZs not much smaller. How much of this is due to economic geography and how much is due to centralising only happening from the late 19th century with Germany and Italy?

[Erpingham]

I think the economic draw for London was, as already mentioned, connectivity originally.  We can forget in the modern age that London was even in my lifetime still a huge international port.  Politics is important because the wealthy will be drawn to the seat of power, so a service economy supporting them will grow and you'll soon have a prosperous middle class, guilds, money-lending etc.

[Imperial Dave]

London centricity is a political/business 1-2 with the social.aspect following..ie you have to be seen to be in London to be anyone of importance

[Erpingham]

London centricity is a political/business 1-2 with the social.aspect following..ie you have to be seen to be in London to be anyone of importance

I always knew there was a reason I'm unimportant

Talking of UK capitals, they are/were all ports.  Co-incidence?

[Swampster]

London centricity is a political/business 1-2 with the social.aspect following..ie you have to be seen to be in London to be anyone of importance

I always knew there was a reason I'm unimportant

Talking of UK capitals, they are/were all ports.  Co-incidence?

Cardiff and Belfast were both chosen as capitals in the 20th century - Belfast since it is the major city. Cardiff almost by accident - it sounds like the secretary of state for Wales said more or less 'Oh, I suppose it might as well be Cardiff'. They didn't really expect that the capital would have much meaning with no devolved government at the time.
With Scotland, if a capital were chosen now and placed in a decent sized city then I guess it would be a port (historical or otherwise) simply because most of the cities in Scotland are by the sea. I presume the Tay was navigable enough for Perth (for Scone) to be a port, though I gather Scone was even less home to the Kings of the Scots than London was to the English kings.

I suppose most of the Atlantic and Scandinavian countries have port capitals. Of the once (and maybe future) countries in the Med, Catalonia/Aragon and the Kingdom of the Two Sicilies had ports for capitals. Belgium's capital was chosen to try and keep Walloons and Flemings both happy. Russia tried a capital near port and went back to the old one. Poland didn't have access to the sea for much of the time when capitals became the thing to have. It is on a pretty big river, as are most of the remainder. There are - what -four? just on the Danube.

[Patrick Waterson]

Italy on the other hand has three very equally sized LUZs around Milan, Rome and Naples. I suppose Rome has the benefit of being central but not much else. Germany also has little difference between Berlin and the Ruhr, with quite a few LUZs not much smaller. How much of this is due to economic geography and how much is due to centralising only happening from the late 19th century with Germany and Italy?

One suspects a lot of the latter, as Italy remained configured around Piedmont/Savoy, Rome (and the Papal States) and the Two Sicilies for longer than it has been a single country.  Germany developed as an integrated whole from 1871 to 1945, and as a pair of rival nations (based on Bonn and Berlin respectively) from 1946 to 1991.  I think politics easily trumped raw economics in these cases.