The sweet spot for gaming monitors at the moment is a 1440p resolution with a refresh rate of 144Hz and above. Games look much more clear and vibrant compared to 1080p, and a higher refresh rate is important if you're playing fast-paced shooters or MOBAs.
Finding that combination used to come at a hefty price, but it's come down a lot in recent times and there's a good amount of choice for under £200. This 27-inch model from Koorui fits the bill nicely, especially with the higher than average 170Hz refresh rate, and thanks to a £50 voucher on Amazon you can grab it for just £140:, and while it lacks dedicated G-Sync and Free-Sync support and fewer connection ports than other monitors have, for pure speed and resolution it's hard to find this combination for cheaper.