Out of curiosity how much are you paying for that i5 2500k? It's still already a generation behind and at least on that ebuyer.com site you linked to, the i5 3570 (current-gen, quite a bit faster) is barely £3 more expensive. With Haswell due out in June, at least you'll be one generation behind instead of two. I'd personally wait for Haswell, but that's your call.
3570 is not that fast, it's around 5% tops. Intel also cheapened on the thermal compound so Ivy Bridge runs hotter than Sandy. Actual amount varies by review, but it goes around 10-20 degrees on stock. This might not be an important factor for OP since he doesn't intend to overclock.
Waiting for Haswell is a good idea, but there are a few factors to consider if you want to jump on release. While the 4570 will debut at around 200$, the motherboards will be more expensive than a similar featured mobo from the previous generation. This is just personal observation from my country, but they're usually at least 1/3 more expensive in the first 3 months.
TL;DR i5 3570 is the best solution in this case as long as the i5 2500 is not more than 5% cheaper.