Steel Frame, just basic cromo, probably an On One Inbred or the like. Nothing fancy, but functional, cheap and long lasting.
Full SLX 10spd drivetrain cheap from actionsports. Under £200 for a full groupset is amazing, but though Deore could probably be had even cheaper, SLX is a massive step up in performance for not much more money.
SLX brakes, for the same reason as the drivetrain, and they outperform anything for the price.
Magura Comp hubs (£40 a pair on eBay right now), significantly better engineered than Hope, and though a bit weighty, they last ages and work brilliantly. Built onto Mavic rims with ACI comp spokes. Mavic still offer the best performance per pound I think, and ACI spokes are just as good as DT but much cheaper.
Thomson post, for reasons outlined above. Would rather spend £50 on something that lasts than £20 on something that doesn’t!
Raceface Ride stem and bars cheap from Merlin. No real reason other than they’re pretty good and nice and cheap currently.
Oh, and Rockshox forks. Home user serviceability (to an extent) makes them a cheaper ownership prospect than Fox or most others (well, rigid excepted, but my wrists are limp!), and you can always find a much better deal on Rockshox than any others too.
So it probably wouldn’t be all that cheap compared to say many off the shelf bikes, but I’d be confident of it lasting well. I’d worry about some of the components on something like a Carrera or a Boardman myself (or lots of others to be fair), as there’s always a couple of weak components on many off the shelf bikes to hit a price point.
EDIT: damned iOS 5, my iPhone 4 just can’t keep up and I keep hitting things accidentally and it posts things before I’m finished!