I would suggest Olympus Health Premium Protein. 5kg is roughly £35. It's a combination of whey/casein/soy to bring costs down but I have had the same results in terms of recovery period/strength and size gains since using it as I had with MyProtein/Maxinutrition/Optimum Nutrition etc.
At the end of the day it's a supplement. Your major protein sources should be from whole foods so spending vast amounts of money on MEGA-HYDROXY-CUT-PSYCHO-MASS with 'Patented Protex technology' containing 'the most bioavailable whey isolate on the market' etc etc etc isn't really worth it.