According to Parkinson's Law it will have to be spent on the Royal Navy. There won't be enough left over for anything else.
And while I realise you're familiar with Parkinson's Law, here's a link for those people who haven't yet had the pleasure.
If the UK leaves the EU in a few weeks time, should we spend the multi-billions we save on rebuilding our Navy (and the rest of the armed forces) or on "social improvement" or a bit of both?