+1 for the cx500 - at scan.co.uk it's £10 extra, but gives you 2xPCIe power connectors, so no need for an adapter. http://www.scan.co.uk/products/500w-corsair-builder-series-cx-cp-9020047-uk-80-plus-bronze-sli-crossfire-eps-12v-quiet-fan-atx-psu.
That's 38A on the 12V rail, which should be comfortable even with a non-crazy OC. At stock the 7870 is limited to 190W, or approx 16A on the 12V rail.
Given your limited budget, I think you've done a good job there at getting the best for your ££s.