There is a difference between a 90-95% ethnic-British country with a mix of a bunch of different diverse countries represented, and a 50% ethnic-British people where the remainder is split into a couple of large ethnic voting blocs who vote primarily in their ethnic interest and live in parallel societies.
I don't know what a slow enough pace of demographic change is needed so that the original host culture can be preserved, but I think it's something measured in centuries and the current rate is much too much.
Does addressing it require mass roundups of brown people? I don't think so, but it does require an acknowledgement that there is a specific native culture worth preserving in this country which is not just the Official British Values of tolerance, diversity etc. That's not a culture, that's how to manage multiple cultures.
I think people often use the term Christian to describe this culture. It certainly is heavily influenced by Christianity, and there is a rich body of Christian-influenced work which requires a deep understanding of Christianity to engage with and which we would be intellectually poorer to discard as our cultural inheritance.
With that said a lot of it is not specifically religiously Christian but is things like the western European marriage pattern - recognisably a cultural factor which distinguishes us from, say, Middle Easterners, but people just lump it in under "Christian" for lack of a better term.