Most Roman migrants to Britain (and other frontier provinces) would have fallen into one of three categories: soldiers stationed there, veterans settled there, and merchants trading there.

Soldiers were of course the most numerous class of Romans in Britain (though I suppose they weren't really voluntary migrants). For centuries, three full legions were stationed in Britain (at Isca, Chester, and York) and large detachments guarded Hadrian's Wall in the north.

Under the Principate, discharged veterans were sometimes assigned land (in addition to their discharge bonus) and settled in colonies, usually near the frontier. Several veteran colonies were established in Britain, of which the most famous is probably Camulodunum (Colchester), destroyed during Boudicca's revolt.

Merchants from other parts of the Empire settled in substantial numbers in Britain's largest urban centers. Little more than a decade after the conquest of Britain, Tacitus could describe London as "a busy center, chiefly on account of its crowd of merchants and stores" (Annals 14.33). Many of these merchants would have established themselves near the legionary camps; soldiers were paid well, and often spent freely.