For 'about 170 quid' the HD787 is your top choice, just check your powersupply has the output and connectors to feed it.
Have a gander here: Dabs, Misco, Pixmania, Scan, CCL, Novatech, Overclockers, Aria and Ebuyer...Amongst others .
you forgot to mention what is you wattage on your power supply (PSU) you will need a minimum of 500 watts for these cards. be sure to check yours.
A decent 450W model such as an Antec VP-450 or an XFX 450W would be more than enough. There's no need to get a 500W model with those for OP's system. Now if OP goes for a 7870 or a 660 Ti, then something with a little more power delivery such as a CX 500 80+ Bronze or an Antec 520W (never get the CX/Builder series models that aren't Bronze because they're older, less reliable designs) will be necessary.