When the census said British White in the title - it doesn't take into consideration non British "Whites". There are hundreds of thousands of French people that live in London. Germans. Swedes. Etc... A lot of them will be "white" (though not all since the term "white" is now such a loose expression). There will be some Americans, Australians, South Africans who are all non "British".
Then, of course, there will be people from all of those countries, who are non "White". As well as people from countries where Caucasian is not the majority.
And then, there are all the British and ENGLISH people who are not "white".
I really don't see the problem with any of the above. All the non British and non English people are bringing trade in, paying taxes and contributing to our society and giving us more. All these cultures, and this melting hot pot of people gives us so much - I LOVE it. Long may it stay.
Also, let's not forget the census was completed by people who are here, more than likely, legally anyway - so all this talk of "illegals" is a little pointless.
Furthermore, the ratio of "whites" may be going down because people have decided to move out of London, or move abroad. What's wrong with that?