It proves nothing I’m afraid, not when you think about the type of individuals involved and how they are likely to have made their money.
How do you know the increased number of billionaires in the UK isn’t simply the result of people switching their domicile to the UK - perhaps getting out of Russia, HK or some other dodgy country - rather than UK born people becoming billionaires through returns on their UK assets/businesses?
Over the past 30 years you’ve had an unprecedented increase in global trade, a far higher degree of global correlation across asset prices and the widespread removal of restrictions on capital flows. Anything the Conservative Party has or hasn’t done in the face of that is largely irrelevant with regard to the number of billionaires globally and where they choose to live. You could of course discourage the very wealthy from coming to the UK but governments tend not to do that sort of thing.