For £50, I would recommend the Cooler Master g series. They are tier 3, so fine for your hardware, and 80plus Bronze rated. AVOID corsair CX(M) and vs series as they are pretty poor quality, but other corsair units are fine, but out of your price range. If possible go for the r9 380, it's a lot more powerful than the 750.
Link for power supply: http://www.amazon.co.uk/Cooler-Master-RS550-AMAAB1-UK-Performance-Modular/dp/B00FNCNWHI/ref=sr_1_4?ie=UTF8&qid=1450479269&sr=8-4&keywords=Cooler+master+psu
Btw 550 watts is MORE than enough, so don't worry about the slightly lower capacity.