First, luxury purchases:
1) Several properties - An English stately manor, with surrounding farmland, village, and railway station, which will be my main base of power - I will bribe planning officials if necessary; A penthouse in London, New York, and Tokyo; a traditional Japanese mansion;
2) I'll keep car purchases utilitarian rather than flashy. Armoured Range Rovers, Bentleys, and an armoured motor home. Security first!
3) A private airstrip, a fleet of helicopters and a custom-fitted Boeing 787, outfitted as a flying 5-star hotel.
4) A modest luxury yacht.
5) A host of servants and staff for my properties and vehicles.
6) Renovation of my stately home to include bulletproof windows, proximity alarms, high walls, security bots, armed bodyguards, and a bunker. If you haven't guessed already, I'm rather paranoid about being assassinated
Hmm, that adds up to about £500 million, not including maintenance costs. Though the stately home could cost a lot more. I'm willing to go up to £1.5 billion if necessary.
Secondly, business empire building, in no particular order or priority:
1) Buyout of BioWare and EA - I will force the latter to break down into its constituent companies, restoring the likes of Westwood, Maxis, etc. and hiring their old staff. I will maintain a 51% controlling stake in all said companies, without impeding their creative freedom. I'd only step in when their management is being stupid.
2) A regional UK railway company
3) Buy up what remains of GAME group or invest in a new videogame store chain.
4) A small airline
5) Square-Enix - Who will be forced to stop making so many crappy sequels, and start making new IP.
6) ATLUS... just stay classy.
7) MADHOUSE animation studios
I don't know exactly how each costs but I should be able to get at least a few companies. If everything goes to Hell I'll just stash away a few hundred million and run for the hills.