I've been doing this as on-the-side work for a few years (it's how I get to play about with new tech so early without having to own it heh)
A good tip I'd suggest is actually register as a company in order to get VAT back (as this is a big chunk of profits), also deal with wholesale distributers when you can afford to. You usually don't have to buy in huge bulk anymore in the UK, which is a huge plus.
Companies like Sapphire (ATI) and XFX (NVIDIA) are both UK based, in-fact both are close to London (don't live too far from either) and they do deals on private purchases.. especially if you do exclusivity deals.
XFX now do motherboards with NVIDIA Chipsets.
Also don't stretch yourself too far with lots of customisation, while people like those here care about that sort of things; adverage punters just want something "cheap" that works.
I mean that's what you've got to remember really.
They don't care what exactly is inside as long as you drop some brandnames.. getting a catchy name for each box is good, and try to make sure that you have set designs for simplicity cause if they do get popular you can then purchase those parts in bulk and know they're needed. As opposed to multiple customisable designs you have no clue if they'll be needed.
Right now I have a deal with a local supplier (RLSupplies) where I get a bit more off their usual prices simply because they know they'll get regular business from me.
