We can define 'replaced' quite straightforwardly: you take a reference point in the past - let's say 1950 - and ask: what is the average proportion of a 2017 European person's ancestry that does not descend from the European population in 1950. The larger that number becomes, the greater the degree to which Europeans have been 'replaced'. (Of course, the answer will depend on where we take Europe's eastern border to be.)

In fact given any two dates and any region you can define a "replacement coefficient" in the same way. For a nation like Luxembourg it's inevitably going to be high over any appreciable period of time. However, I'd expect that the replacement coefficient for Britain between (say) 1066 and 1950 was less than 0.1, given that the replacement coefficient between 10000BC and 1950 was only around 0.2.

In Britain, the replacement coefficient between 1950 and the present is probably larger than between the ice age and 1950, and is increasing rapidly.

Replacements happen all over history - it's not like the sky will fall if Britain, or Europe, gets (say) a 0.8 replacement over (say) the next two centuries. I will not pretend to be indifferent about it, though. I don't think it's going to be good for our descendants, or for the world, if they are minorities in their own nations.