I'm afraid the best place to buy Amiga equipment in the UK is probably www.ebay.co.uk
I don't think anyone in the UK sells brand new 68060 accelerator boards anymore which is a pity.
Or you could also try www.amibench.org
which is a good place to buy and sell Amiga equipment in the UK. It's not quite as safe as eBay though. The 060 accelerators usually go for between £120 and £180 on eBay. Good luck with your hunt!
The only shop in the UK that I know of that still sells hardware is www.forematt.co.uk/amiga.htm
but their stock of hardware is extremely limited. They're more of a software company.