The only monitors that'll come in under £300 and are great for gaming are the Korean Crossover monitors. My recommendation would be the Crossover 27VV IPS QHD Freedom for
~£270 shipped on eBay**. This biggest drawback is the lack of a fully adjustable stand, but it does have a VESA mount in case you want to replace the stand.
(** International items may be subject to customs processing and additional charges.)
You can read the recommendations from this monitor enthusiast forum post:
Best 27" 2560x1440p Monitors: AHVA/IPS/PLS (Crossover 27S -> 27V -> 27VV). Going down the list, it's the highest rated multi-input monitor within budget:
Multi-Input Matte: Gaming Priority=Left # / Image Quality=Right # Model : (Current Price on Amazon.co.uk)
#1 / #2
Eizo FS2735 : (
£771.10)
#1 / #2
Viewsonic XG2703-GS : (
£738.97)
#1 / #4
Acer XB271HU : (
£629.99)
#2 / #4
Asus PG279Q : (
£689.22)
#3 / #3
Asus MG279Q : (
£472.48)
#3 / #2
Crossover 27 Fast 144 : (eBay
£361.46)
#3 / #1
Crossover 27V & 27100Q : (eBay
£268.65,
£303.35)
#4 / #2
LG 27MB85R : (
£475.03)
#4 / #1
Qnix QX2710 Multi Hot : (eBay
£204.90) (overall quality is lower on newer models)
#4 / #2 Asus PB278QR/2014 PB278Q : (
£341.48)
#3 / #3 AOC Q2770PQU & Q2775PQU : (
£599.00 &
£334.99)
#5 / #2 BenQ BL2710PT : (
£369.92)
#6 / #4 BenQ GW2765HT : (
£291.99)
While the BenQ GW2765HT does come in under £300, it seems to have
fairly high input lag, so it's not the best for gaming [Total Display Lag (SMTT 2) = 24ms]. The QNIX QX2710 is only slightly faster (23ms) but much cheaper.
Hopefully someone with experience with these Korean models will chime in.