You may be better off getting a better quality pair second hand rather than hoping for a pair that will satisfy you for £50. I'd certainly recommend 8 x 42 for general birdwatching, that's a perfectly adequate magnification and light allowance at the objective lens end. x 50 would be heavy to hold and are OK for seawatching where you need a wide angle view. The heavier the binocular the harder it is to avoid shake and your arms/neck will start to ache if you have to lug them round for too long.
Try these sites:
Used Optics at Birdnet Optics (the RSPB binos are generally good value)
Binoculars
(the above link gives some good basic recommendations as well as being a reliable second hand optic dealer)