Why the SLI mobo if you're not a gamer/user of SLI? Maybe can save a few £ by getting a non-SLI mobo.
Same with the video card... I know Vista will need video power, but it may not need that much. But then again, that's not that bad of a price (doing a rough conversion to US $ by £x2). Umm, the 2nd price is with VAT, right?
Don't know if you can get it there, but Benq & Nec make darn good burners - might be a better price (or not). Just throwing out options.
Are you overclocking? If so, try to upgrade your ram a little - either faster (PC4000-ish?) or lower latency (lower latency would improve performance at stock speed too). But don't go overboard - 3-3-3-8 to 2-2-2-5 only gives a couple % more performance. Not worth it to me, but worth it to some.
Tagan makes good PSU's & that's good wattage. No clue on the case - I like cheapies (don't know if you get £10 cases over there, but the ones I always buy are about $20US.), so I stay out of the case debate.
Only other suggestion I could see is, even though you didn't mention needing it, is an X2 if your budget can afford it. It will make it just that much longer before you need to upgrade again. The X2 4200+ is like having 2 of the 3500+, so performance will be just as good.
err, you should either go for 3700+ in single or x2 3800+ in dual. Why are you suggesting PC4000 rams? he's not gonna OC. I'd say some GSkill Value ZX or OCZ El Platinum ddr400 for low timing and high OC.
Nice system overall, but I will 2nd(or 3rd) what the other posters said. Can the motherboard you picked and get the Asus A8N-E instead. Its around 50 pounds if I remember correctly. Also, I would go for the X2 3800+ or Opteron 165. X2-3800+ for a good dual core system, Opteron 165 if you are going to overclock it. You will like the system when you build it! Enjoy.