Just fitted an ikea over chrimbo. Not that much difference in price to the B&Q off the shelf stuff but i think it's better quality, easy to put together.
B&Q really stick it to you on the extras like trim etc. which adds up.
Only thing with ikea cabinets is the backs have to go flush with the wall which means you have to be a bit creative where there is plumbing. Would get your handles, sink, taps and stuff from somewhere cheaper though.
Their worktops are wide though and their wall cupboards a lot deeper than standard.