I wouldn't bother upgrading, it sounds like you just have money burning a whole in your pocket, rather than a "My PC isn't doing xyz, and I'd like it to"
Upgrade when you want to do something that your PC can't do, or when it feels slower than you'd like.
How about spending the money on a home server instead? You could do it for a hell of a lot less than £2k, but it would be an interesting, different-to-your-pc project if you've got an itch to play with some hardware? Plus it could actually fulfil a purpose
Other than that, a nice AV Receiver and decent 5.1 or 7.1 sound setup would be a great upgrade: good sound can really improve the gaming "experience", plus music and movie watching. £1000 will get you a nice setup here.
Then next year grab a 4K monitor and a AMD 300 series card, and job done. Your PC will be fine for at least another year or so.