Ok, so where could I get 8GB for my iMac in the UK and online ?
Good question. I don't even know where one would get them here in Japan! I have yet to see even 4GB DDR2 SO-DIMMs much less 8GB. There do seem to be some specialized brands of desktop RAM in that configuration available though.
It will still be cheaper than buying a Mac Pro.
Not necessarily. The abovementioned desktop parts I mentioned start at about $1000 US each.
Ideally, if you have need of more total system RAM you're probably going to be better off with a system that has more memory slots for smaller denominations (like a Mac Pro). Even when those larger-sized parts hit the market they won't be cheap.