Ok great... As I said, the first thing that is going to dictate your purchase is your soundcard.  It's probably only compatible with certain chipsets or what have you.  You're going to be using Focusrite Saffire LE.  A bit of google turns up:  
http://www.focusrite.com/answerbase/en/ ... php?id=334  -  I'll save you the read and quote:
Check the chipset and type of FireWire card. We recommend FireWire 400-only, non-combo (i.e. not with USB AND FireWire ports) cards with either a Texas Instruments or a VIA chipset, from a known brand manufacturer such as Belkin, Lacie or SIIG.
Other chipsets and models also work, but the above specs will give the best results. For more information see FireWire compatibility article.
Also, a bit more rummaging turns up the fact that when using this card in windows 7, you'll have to roll back the driver for the firewire controller.  Again, not a big deal, step by step instructions provided on the focusrite website.  So, knowing that everyone prefers ASUS, we need to find an ASUS motherboard that has a Firewire 400 port with either a VIA or TI chipset.  (Sorry to sound like a technical douchebagg, think this post might be helpful for others)  Otherwise we can use any motherboard we like - but we will have to buy an additional firewire card that is compatible with our audio interface.  I've done the dirty work for you.  
Here's what I would do if hired to build a DAW based on your parameters (price).
http://www.newegg.com/Product/Product.a ... 6813131665 - 
Motherboard - $190 - ASUS Sabertooth X58 LGA - This has a firewire 400 controller with a VIA Chipset (I know this because I looked it up on ASUS website)  It's actually a kickass motherboard, USB3, SATA 6Gb, etc, all the new shit.  Also, this motherboard supports multiple RAID levels, and I believe its crucial for all DAWs to run at least RAID 1.  So technically, you won't have to buy any other cards. (like firewire or SATA RAID card)  This is everything you need. Includes Fan XPert Software, this time quoting from ASUS website.  
Fan Xpert
Active Quiet & Cool
Fan Xpert intelligently allows users to adjust both the CPU and chassis fan speed according to different ambient temperature , which is caused by different climate conditions in different geographic regions and system loading.Built-in variety of useful profiles offer flexible controls of fan speed to achieve a quiet and cool environment.
I have Q-Fan, which is an older version, and it works VERY WELL.  It makes you look all sophisticated n stuffs to your non technical friends.... I turn down the fans during mixdown.  If I have to record, I simply record in a different room than my PC is located.  Of course, not everyone has that option.
http://www.newegg.com/Product/Product.a ... 6811129049 - 
Chassis/Case Fans/650W Power Supply - $249 - Had this case for 3 years, absolutely love it.   See previous posts.  I know that this chassis is an ATX chassis, meaning any ATX compatible motherboard will mount into it.  ATX just means basically the size/shape/layout of the motherboard.   How you're going to mount it in the chassis.  The above motherboard is an ATX motherboard.
http://www.newegg.com/Product/Product.a ... 6819115211 - 
Processor - Intel Core i7-950 Bloomfield 3.06GHz - 200$ - the de facto standard?  
 
http://www.newegg.com/Product/Product.a ... 6820145258 -
 6GB DDR3 Ram -$150 - Corsair RAM is nice.  This isn't the fastest RAM that this motherboard will support, but its damn nice and plenty fast too.  If you've got a bit more budget, I would get faster ram than this.  This is DDR3 1600.  The motherboard supports up to DDR3 1866.  Upgrading to the fastest RAM would add $50 USD (30 pounds?)  to the price for the same amount, 6GB.
http://www.newegg.com/Product/Product.a ... 6822136533 -
 2 x Western Digital 1TB Hard Drives (Giving 1TB total storage - redundancy) $200  -  SATA 6GB/sec   --- 10,000 RPM is nice, but we get into the price/performance discussion again.  If you can afford two 10,000 RPM hard drives and still have enough storage - do it.  I would spend the money here over quiet fans or the biggest baddest processor out there, and even over faster RAM.   It's easy to add hard drives later, even to a RAID array, so this and the RAM (and processor) is where you can skimp now, and upgrade later.
http://www.newegg.com/Product/Product.a ... 6827136180 - 
LG DVD Burner, SATA - $18 - How are you gonna install windows?
For a total of approx: $1010 or 1,010.00 USD 	= 	652.686 GBP - Right around your price point I think.  I would LOVE to have this system, unfortunately an engineering student with a mortgage to pay can't afford these luxuries.
I hope you aren't including Mouse/Keyboard/LCD Display in your price range.  Anyways, mouse and keyboard is 10 USD for both (MicroCenter is the shit for for my fellow yanks - but they dont have locations in all states)  I would use a CRT with this system for a few years if I had to.  
To DZA: Don't be intimidated if you haven't assembled a PC before.  I would highly recommend doing it yourself, even if you don't save any money.  This way, when something goes wrong, you'll know exactly what hardware/drivers you have, how its assembled, etc.  The only thing you could really mess up on is when you connect the power switch to the headers on the motherboard.  If you put the connector on backwards, you will short out  (brick/fry/ruin) the motherboard as soon as you hit the power button, at least on old ones.  Learned the hard way, twice.  I can help you with this when its time, or whatever.  Other than that, assembling a computer from parts is a lot like kindergarten.   Match the shape with the same shape, match the color with the same color.  It's nearly impossible to put stuff in the wrong way in 2010.
Secondly - don't build this kick ass computer I've picked out for you and then ruin it by going on the NET.  You DAW should be for production only.  Use the computer you're on now for the internet.  If you have to use this kickass PC for something as lame as the net, download and install a copy of Ubuntu AFTER you install windows 7, with a small partition, say 10 GB.  It's easier to install than windows - works right out of the box.  Then, only go on the net with Ubuntu, not with windows.  Ubuntu is WAY more solid when it comes to firewall/antivirus/spyware.  And its a hell of a lot faster.
Best advice I can give to anyone trying to produce great music:
GET RID OF THE INTERNET
I only have it at work/school.  Not at home, not since 2004.  Now I only have Ableton Live with every imaginable VST plug and many hardware pieces to pass the time.  Loving it always.  When I had the net, I would always be paging between Ableton and things like DSF.  Fooking productivity mass murderer is what it is.
Any questions? Anyone?
Dude! I just spent like 2 hours researching and typing this... do I get extra credit?  Too bad I'm so far along with my courses,  I probably could've turned this in for a project a few years ago 
