The SeaSonic SSR-550RM runs around £75. It's a gold cert PSU so it's more efficient and it has two 8-pin PCIe cables to power even the most power hungry GPUs.
The XFX P1-650X-XXB9 is 650W and has four 8-pin cables so you could run two heavy draw GPUs if you wanted. It's around £70. Or you could go with the little brother, P1-550S-XXB9 for around £50. It's 550W and has one 6-pin and one 8-pin connector. That means dual-GPU's isn't an option, but you say cash and it's enough to run all but the beefiest GPUs.
Look for PSUs made by Antec, be quiet!, FSP Group, PC Power & Cooling, SeaSonic, Siverstone, and XFX. Check the PCIe connections and find one that has at least an 8-pin ( often 6+2-pin, ) and one 6-pin connector.