For that budget you're looking at an rx460 (~£110) or rx470(~£175). Both would be an improvement on your r9 270.
If you can scrape together a few more £, you could go for the excellent rx480 (£200+), it's more expensive but moves out of budget card territory into the realm of mid-high end.
If you want to switch to the green team (nvidia), you're probably looking at the gtx1050ti (~£150) or the gtx1060 (~£200).
My personal choice would be save a few extra £ and grab an rx480, preferably the 8gb version. While nvidia holds the current high ground for performance, they are expensive. AMD's rx480 is the sweet spot for price/performance right now.
