My ex, who was a kitchen fitter, says:
Magnet is good but very expensive.
B&Q are cheap, sturdy and an utter b*tch to put together (if you're trying to do it yourself).
IKEA range from cheap to middle of the range, has a good selection, but the units you want are often out of stock.
Wickes do a good range, but he has no experience of them.
MFI have gone into Administration.