I wouldn't be happy myself, but there isn't much you can do about it I'm afraid unless you get a very sympathetic ear from whoever is dealing with your RMA.
Going from past cases from various posts on the matter, Gigabyte are within their rights(and could possibly even say they are being generous if you got it via OCUk's cheap offer last year) to offer you the 570 as a replacement due to depreciation.
I realise it has less vram, but the performance is very similar.
Consumers rights in the UK:
'If a repair or replacement isn’t practical
A trader doesn’t have to offer you a replacement or repair if:
it’s too costly for the trader'
'If this happens, the trader should either:
offer you a partial refund if you return the item (to allow for the use you’ve had from it)
let you keep the item and give you a reduction in price for the fault'
http://www.direct.gov.uk/en/Governm...merrightswhenbuyinggoodsandservices/DG_194650
I think the reason Cleeecoo got a 580 as a replacement was because his 480 Soc was faulty on purchase or within a fortnight or so at the time, iirc.