Lots of these oligarchs have British residency though. So they may not be citizens but have the right to use our law systems. But it is super complicated, I agree.

That was the case until 2010, when Radmacher v Granatino was heard and the Supreme Court upheld the prenuptial agreement. http://www.bbc.co.uk/news/uk-115809… They were careful to limit the decision so as not to recognise a blanket right to have prenuptial agreements upheld, but the door has been opened.