$15 billion.......... hmm well for starters I'd give some money to my family and friends and see them OK for the rest of their lives.
I'd then emigrate to far sunnier climes and buy a huge plush pad with loads of stuff, several top of the range cars and a boat. I'd have a shopping spree on clothes and electrical goods. I wouldn't need a maid, butler, chef or whoever, I've spent all my life doing all the odd jobs, the cooking, the cleaning, driving around and so on, I'm still going to do that myself. I wouldn't buy several properties in other countries at once, I'd rather spend the dosh on lavish holidays instead.
I'll also renounce my citizenship and hand my passport into the UK embassy and tell them to shove it and the union rag flag up their jacksie, best £350 (or however much it costs now) I'll ever spend. Then stick the rest of the money in the bank and live off the interest!