thanks ionutd and headtrauma for the help! i've got some updates. so i ended up buying a rx 550 for £60. mainly because this is what i have in my own computer, and was able to get it new from a uk retailer for what i think was an ok price.
i installed it, and games were reportly showing good fps. however the computer felt slightly slower, and the internet connection was a lot weaker despite showing 5 bars in the taskbar. i removed the gpu, and these problems went away. reading online, it sounds like it could be a psu issue. so i've bought one of these, and will feedback in this thread with the results