Some introduced species to Britain by the Romans include:
Brown hare, Roman snail, Peacocks, guinea fowl, pheasants, domestic cats and possibly fallow deer.
…vegetables such as cabbages, peas, celery, onions, parsnips, leeks, turnips, cucumbers, radishes, carrots and asparagus, as well as fruit, including plums, pears, grapes, apples and cherries and nuts such as walnuts.
In addition, they brought over lilies, violets, pansies, poppies and the (somewhat less pleasant) stinging nettle
see The Archeology of Animals, Simon Davis