Unfortunately, the future tanker won't be owned by the RAF, but leased and in part operated by the owner. Which means that the cheapest package will probably win, with the winner being not selected wholly on suitability for a tanker. The way I see it is this: (All IMHO)
767 - Good basic technology, AFDS can be used for AAR with minimal modification, right size, needs to carry more fuel.
A330 - V big, perhaps too big for existing tanker airfields and forward operating bases, wing pod fitting problems with MLA (akin to the L1011-500s active wing relief), autoflight system not compatiable with AAR without expensive modification, carries lots of fuel (in proposed tanker guise).
I gather the A300 isn't in the picture. And as for the engines and certification, that won't be a snag for hte RAF as the package deals with all that - initially the contractors' problem. Where the aircarft will come from will be the contractor's problem (initially) as well.