Got an absolutely awesome build lined up - graphics tablet included (although I don't know if it's any good), dual monitors, fantastically powerful.
I checked it against a few different well known companies, and, well, let's just say they didn't come close to matching it in either performance, price or overall quality. It came within your £5k BUDGET, and could easily be cut down to save a TON of cash if you like, whilst still remaining an extreme performance computer.
I'm feeling tired right now, not really up to posting all the links and doing the tags etc, but I'll post it tomorrow when I've checked over it once again, checked prices, maybe researched graphics tablets a little. Someone may need to poke me and remind me if it doesn't appear.tablet link? Wacoms are the ones to getI figured Wacom was a good brand, as that's all that was on offer at this specific online store.
If it's no good (I picked one that looked good but as I say I know nothing about them) then it can always be discarded.

Well, here goes a build:
Intel Core i7 920 CPU - £241.49
Gigabyte GA-EX58 Extreme motherboard - £287.49
2 x Corsair 6GB (3x2GB) Dominator 1600MHz DDR3 RAM - £377.18
Coolermaster CM Storm Sniper case - £137.99
Be Quiet! Dark Power Pro 1200W modular PSU - £218.49
2 x Acer G24 24" WIDESCREEN LCD - £611.78
2 x BFG Geforce GTX 295 - £873.98
OCZ Vertex 120GB SSD - £390.99
WD Caviar Black 1TB HDD - £98.89
MS Windows Vista 64 bit Ultimate - £155.24
Wacom Cintiq 12WX LCD Tablet - £862.49
Samsung 22x DVDRW drive - £16.09
Thermalright Ultra 120-Extreme Black CPU cooler - £40.24
Thermalright 1366 Bolt-through kit - £8.04
OCZ Freeze thermal PASTE - £3.44
3 x Noctua NF-S12 fans - £41.70
Akasa 22cm fan - £12.64
Logitech G15 keyboard - £57.49
Logitech G5 mouse - £35.64
Everglide Titan MonsterMat mousepad - £18.39
Logitech X-5500 5.1 speakers - £187.44
Sennheiser PC350 headset - £91.99

Total price: £4798.74 +shipping (not totally accurate as the WD HD is out of stock, but around £32)
There are plenty of areas to cut back a little and save a lot, and also areas where you could push a little further.
I'm checking around a few manufacturers, and they're still not coming close.

No idea if this will persuade you (the OP) to go custom built or not, but I gave it my best shot.
As always, everyone please feel free to question my decisions, ask why I chose "x" component, or suggest improvements, I'm always willing to listen and learn.That tablet is great! I want one of those cheap wacoms with no screenSorry such slow reply on my behalve, have been all over the place with this 3/4+ snow I got going on around where I live, novelties worn off now so hopefully it should go soon.

Anyway, I need to look into the product thats I am capable of understanding (dual monitors, graphic tablets, few other bits and bats that to me and the work I do matter if you get my drift) the rest I'd prefer to sit down with something who is genuine and honest and tell me what the script is with the individual product, the advantages, disadvantages etc. I am not sure my direction, I don't think I'd like to deal with a website that does the build for you with all the separate parts as my knowledge is very little in certains parts, so poss going to a local independant company rather than a chain store that refuses to give me discount if I am spending a good couple of grand with them.

wish i had 5k to play with Joshuashaw, we can explain a lot of things here, if you ask us about the individual items you'd like to know more about.
Other than that, there are a lot of reviews out there on most products, it's just a matter of finding a trustworthy source that gives the information on what you need.
Local INDEPENDENT stores can be good, however things to consider would be the quality of the after-sales support they give you and how much experience they have in custom building systems, especially high-end systems. Also, if they're quite busy your build may get delayed by quite a bit.
Just some things to think about anyway, I'm sure you've already gone into this but it doesn't hurt to have a reminder.