Honestly... if I had 15 billion I'd probably start funding projects for the government just for fun.
Can't afford two aircraft carriers? I'll get that for you... No I insist...
If i had 15 billion, the last thing ide do is buy UK gov aircraft carriers, the useless sods would only go & sell them off for peanuts, no, if i had 15 billion, ide do a Bill Gates & set up my own charitable foundation, stick 14 billion into that, & keep 1 billion back just to stop myself from feeling poor, but put the rest into doing some good in the world.