Hi Mike,
If I were you, I would go for a Radeon 6850 or one of the factory-overclocked GeForce GTX 460 768MB.
I was looking at the UK prices (and oh man are they high compared to Canadian prices in term of pure currency conversion) and here are some of the prices I found
Sapphire HD 6850 1GB GDDR5 Dual DVI HDMI Mini Display Port Out PCI-E Graphics Card 150 GBP
MSI GTX 460 Cyclone OC Edition 768MB GDDR5 Dual DVI Mini HDMI Out PCI-E Graphics Card With H.A.W.X. 2 Game Voucher 130 GBP
**EDIT**: Just saw infernox_01's reply and seems scan.co.uk has better prices than the ones I listed
So here's the MSI 460 GTX 768MB OC Edition for 126 GBP (This one is slightly cheaper but does not come with the voucher, so if you don't care for the game.. you can shave off a few pounds)
http://www.scan.co.uk/Products/768mb-msi-gtx-460-cyclone-oc-3600mhz-gddr5-gpu-725mhz-shader-clock-1350mhz-336-cores-dvi-mhdmi
And this is actually a much better HD 6850 version (custom cooling, huge overclocking potential) at 149 GBP
http://www.scan.co.uk/Products/1gb-asus-hd-6850-oc-4000mhz-gddr5-gpu-790mhz-960-stream-processors-dp-dl-dvi-i-dvi-d-hdmi
**EDIT_END**
Both those cards are better than the 5770. The 6850 will perform better than the GTX 460, but it is 20 pounds more and a bit over your budget.
If you can afford the extra 20 pounds go for the 6850 because right now it is one of the best performance/price cards with alot of overclocking room (Some of the GTX 460 1GB OC edtion will compete very with it though).
But, if you absolutely want to stay in-budget, go for the 460 GTX linked here. The MSI OC Edition is one of the better versions out there in my opinion.
Oh, and I almost forgot .. the GTX 460 comes with a voucher for HAWX 2! So, if you don't have that game and might like it .. this is even more added value there!
Both of these should satisfy all your video/gaming needs for a year or two, unless you like to game at very high resolutions max settings full AA...
Hope that helps.. and happy hunting!