In reply to:

Well, the Romans were in Britain for about 400 years, and they did have an impact on the languages spoken on the island. There are many Latin loanwords in Welsh, Cornish, Breton (off the isle), and Irish.

In the case of Irish at least, it was ecclesia and not imperium that left a legacy of Latin loan-words, and in fact any infuence on Irish dates from after the withdrawal from Britain. The influence of Latin is most evident in vocabulary that relates to church (eaglais) and school (scoil).

It's worth noting that the Bretons were actually on the island during the Roman occupation, and subequently migrated to Brittany.