You already have a new-ish CPU and an SSD, so I would suggest a GPU upgrade. For £300, you could get yourself a decent GPU upgrade like the 3GB XFX Radeon HD 7950 Double D GPU, which is £220.55 at Scan.
This is way under your budget, and even if you wanted to spend more, you can go for the 3GB ASUS Radeon HD 7950 DirectCU II TOP V2 for £304.86. Obviously this is a little over budget, but it is ASUS' best HD 7950 GPU and has some insane overclocks and an even more insane cooling system.
My preference would be the XFX GPU for £220.55 and you could spend the extra money saved on a bigger/faster SSD maybe?