This is a very well established and reputable buyer and seller of precious metal, based in London.
I don't know if you shop around, whether you can get a better deal on sovereigns, but they're worth knowing about as a point of reference.
They purchase metal at their warehouse in Gallions Reach.
They are a member of the LBMA and will sell silver to you and store it onsite so as to avoid a VAT charge. Personally I would rather gouge my eyes out rather than buy and store silver with an LBMA member, but there you are.