OK I am not great a pound to dollar conversions but I have upgraded a pre-built comptuter before.
1. Avoid the low-end ATI cards like the ATI 5450 like the plague - they are junk (I own one so I am not throwing rocks at anyone)
2. The minimum ATI card would be the 5770 but if you can swing it the 6850 is much better. The GTX 560 or even better the 560Ti are great cards for the money. If you are going to game, these would be the cheapest options available that still have some bang for the buck.
Tom's Hardware's recommendations are here:
http://www.tomshardware.com/reviews/best-graphics-card-gaming-performance,3042.html
I know you are on a tight budget but if you can swing a 6850, 6950 1 GB or a 560Ti, you will be in the best world for gaming on a budget. These cards are only about $50 more than the cheaper and much less powerful options. The newer games will demand at least the 6850 or the 560Ti power to really enjoy them. I have heard good things about the 5770 but it is about $110 and you can get a 6850 for around $150 and it is much more powerful. The 560Ti is approximately the same price or a little more and it is a very good gaming card. With a 100 pound budget, you are very close to a 1GB ATI 6950 which would be your best choice (about equal with the GTX 560 Ti)
3. For a PSU, you could try the Corsair TX 650. Right now it is $65 after rebates on newegg (yes I know you probably have European computer supply stores - not newegg) which is somewhere around 30 pounds. Check you local UK suppliers
4. For a CPU cooler the Coolermaster Hyper 212 Evo is within your budget and is a capable CPU cooler. The Titan Fenir is also good but it is more expensive - Bit Tech regularly recommends the Titan Fenir (check their reviews, they are based in the UK).
5. Make sure your motherboard has an open PCI-E slot for the new video card (presumably you remove your old one but some computers just have PCI slots, not PCI-E slots)
6. The Corsair H50 is not that great a cooler. Look at the H60 and the preferably the H80 for pricing if you want the water cooling. The H60 and H80 are much better performers than the H50. You might do a quick check on the Noctua NH-D14, it is an amazing air cooler for about $80 USD (about 45 pounds approximately). the Titan Fenir is available in the UK and it is cheaper and it is a great air cooler. BE CAREFUL with the NH-D14 it is very large and it may not fit in your case. The Corsair H60 or H80 need to be checked also to see if your case will accommodate them. The Titan Fenir is smaller than the D14 but you will need to check it also.
Good luck with your new computer