I have done some more digging since. It seems a lot of shopping around is needed, but that either I would need to sell in shops that are dedicated to buying and selling coins/bullion or using online sites that will buy any gold at often very competitive prices (they provide you with a secure envelope that is insured for the contents).

No concerns with selling gold coins as they seem to be free of any capital gains tax, oddly this does not apply to silver coins.