I was reading your budget as £20 for a case _and_ a power supply - i.e. £10 each. Maybe I read that wrong and you meant £20 each. Doesn't change my advice since every single no-name PSU I've ever had is now in the bin and every single Antec/Enermax is still going or has been outpaced by power hungry components. I've also learnt from sliced fingers that getting a case that's been designed rather than thrown together is much more useful than getting blood all over everything.