I'd recommend the Corsair HX 850W. It's showing $129.99 here in the US so I think it's pretty close to your price line. I have the 750W version and I've been completely happy with it.
You won't be able to get a fully modular PSU for under £80. Cheapest is the Corsair RM850 which is a great PSU. 800W is overkill for this type of build anyway so I would recommend the RM550 - fully modular and it stays in the original £80 budget. http://www.uk.pcpartpicker.com/part/corsair-power-supply-rm550