Sounds like to me that you have got your heart set on buying the CA851A amp.
If this is the case, see if RS can either offer you a free home return demo or they can give you a in store demo connected to the most expensive speakers they have on display in the shop, to give you an idea how good the amp preforms.
Can't comment on the KEF speakers, because I don't no anything about them.
if you are seriously looking at the CA851 amp, then personally I think you need to be looking at £1500 plus for brand new speakers to do the amp justice.
Or you could save some money on the speakers and go down the second hand route somewhere between £500 to £1500.