they didn't really make a big effort to get here, Claudius just wanted some feeble little island to conquer so he could have a triumphal march through Rome. It became quite a wealthy outpost, though, They left when the troops were needed nearer to Rome. For all the centuries they were here, their influence vanished quite quickly. When London was resettled by the Saxons, they ignored the Roman settlement in the City and set up shop further upriver. The language faded away too; what we have now is part Angles and Saxons and part Norman French.