Obviously the answer is no, but the huge increase in cases in London should tell anyone it's not a good idea for Xmas mixing. How high will the rates go before he has to step in?
I think rather than cancel Xmas for the worst hit areas the only thing he would do is cancel Xmas mixing for everyone and do a complete u turn rather than single London out.
If however the North west still had high rates he would have no hesitation In stopping the north west mixing at Xmas but allowed other areas to mix.