1) With the current premium exchange rate of pounds sterling over US dollars, I can't imagine anyone saying that it's easy to find comparable prices on components in the UK -- even here in Ireland, wholesale prices are exorbitant by comparison
2) So take my advice and build your system in the US (with the best quality components you can afford). You can RMA some North American compents (harddrives, frinstance) to European tech support for repair and replacement. The good news is that good quality power supplies will work fine with the European 220 watt voltage -- you just flip the ps switch and swap the power cord. However, I'd advise you to get a serious power converter in the UK so you can run North American appliances with integral cords and fixed wattage. I've got a 400 watt ps that handles a lot.
3) Get a really good quality surge protector or line conditioner in the US to run North American components off of -- connect it between your convertor and the compents.
Finally, be canny when you take computer stuff through customs. You are allowed to take a max of £275. (bout $400) into the country before you will be charged heavy Value-Added tax and duties. Thus:
A) Take all components that are separate (including printers and monitor) out of their packaging and boxes. Driver disks and manuals go into ziplock bags that get put all together in some piece of stow-on luggage. I usually repackage individual components (kept in opened and resealed antistatic bags) taped up and carefully packed in one onboard bag -- that way, I know nothing's going to get smashed by a handler.
B) Once a computer or component has been taken out of its packaging, it's used. You should set your own estimation on the value of used computer equipment. Most people at customs do not have a clue; however, if you ship or mail material by courier, and you insure it for it's full value, Customs will appraise the product's value by this figure.
C) If a customs officer asks you at the airport what 'this' is, it's not enough to explain that it's your 'personal' computer: 'Its value is around $1000.' you say, 'But it's only for my use -- not for resale.' 'Well' they'll reply, 'There's a $400. limit on what you can import over here, so, if you'd care to step aside with your cheque book, we'd like you to indemnify this against any future resale.' Hence, you should say, 'this is an old used computer my Uncle gave me, when it got so archaic that his office couldn't even use it any more. It's got a combined value of about $200.'
I have paid through the nose, and, with experience, I have avoided payment. Good luck.