Just recently I was looking at leftover Roman structures in Britain on google, many of which are overgrown and crumbled. This made me wonder how much the Romans really settled or mixed with the native Brits. Were they more like the Normans, perhaps, who seemed to mostly only really make a difference amongst the nobility (to my knowledge).
Do we know for sure if any of the Romans really settled in Britain (as in, settled the land with their families)? I'm part English, for example, does this mean that there's a marginal chance that I would have ancestors from England who were once Romans?
You've got to be careful about how you use the word "ethnic".
Strictly speaking, ethnicity is about social groups and culture including language, religion, art and so on. There is no doubt that they left a large footprint, founding many of the major cities, building widespread public works, and hiding coins in fields to delight future numismatists.
But.... You seem to be looking for the genetic footprint instead.
The answer is that the genetic impact was much smaller than many expected. Earlier this year a genetic study of the British found that Roman, Norman, and Viking influence pales in comparison to the influx from northwest Germany. The full article in Nature is here.
(Insert joke about waiting 500 years to be introduced).
A different study, following Y chromosomes estimated that 4-8% have at least some genetic markers originating in Italy, although the article cautions that there is no telling that it happened in Roman times, or later.