If you're going to be buying a new monitor with the future in mind, 4K is the way to go. 4K monitors can be expensive right now, but eventually the price will go down and they'll become the new norm. Right now on eBuyer, you can get a very reasonably priced Acer RT280K 4K monitor for £225.
It's a 28-inch display with that 4K resolution of 3840 x 2160, and these are pretty good specs for the price. There are a couple of drawbacks though. It's only got a standard 60Hz refresh rate, however it does have AMD's FreeSync anti-screen tearing technology support. The other drawback is the panel type, as it's a TN rather than IPS, which is what you'd normally want for gaming. That's not to say TN is completely useless, but you just won't get as nice colors or viewing angles.
The price of £225 is a pretty big discount, as the Acer RT280K often sits above £300. It's dropped down to this price before, but only very briefly, so if you're interested you might want to take advantage of the price quick. It's also on sale at Amazon right now, but eBuyer's price is £20 cheaper.