1)the largeness of a Basilica isnt an indication of Importance in any regard; in Britian alone cities that were important to the Roman can be summed up more by strategic importance at sites such as Deva, rather then the provincial capital; by any stretch of the Imaginiation, London was even on a top 10 of cities Important to the empire, and in comparison to cities such as Rome itself, Alexandria, Antioch and Mediolanum, it fully pales in comparison- Londinium is only regarded as important by todays schoalrs because Britian rose to primacy in later years, if it handnt, it woudl be just as reltivlly unkown as Aquincum, another town that dwarfed Londimum in every respect, that would go on to be modern day Vienna, Austria.
2)as far as the defenc eof Britian goes, you must remember that even before the intial invasion of Britian Roman citizens were living thier as merchants, and once the invasion got underway; spawned by the political need of claudius to pick out an easy target of invasion to prove himself, and the real need to make sure to quel any indipendence movement the Britianic celts might spawn on the mainland, a good government like that run by the principate is going to have a very hard time trying to justify the invasion, and susquent abandoment of a hige investature of money and lives- once the Romans got to britian and conqored it, it would be a hige sign fo weakness to abandon it.
3)you lis tof legions is skewed at most only 3 legions ever served in britian outside the intial invasion at once, unless it was for a specifric campaign, and and the Danube frontir for instance dwarfs it in terms of military importance, and the numbe rof active legions stationed it bears this out.
London wasnt an important center, unless you consider importance to the locals a real credential to saying a city is of international and lasting importance; by Contrast Rome from its earilest days was in contest between local powers, and relitivlly quicklly after its indipendence was making international treaties with the like sof the Carthaginians.