Dr. Rox, what about Morocco? Its getting close to Spain and shares the ferry link between Tangier, Morocco to Algeciras, Spain. Algeciras is few kilometers away southeast of Seville.
TAR has been on that ferry in TAR 3, IIRC, and it would be awkward with their filming schedule to have teams in Rome, travel to Tangier for one leg, and then take a ferry to Spain. I'm not sure that is the way production would do it now except as a substitute leg. The pattern in the past ten seasons or so is to plant legs along the way back to the US.
I would not assume that the 11th leg was in the UK (as it was in season 22, btw) simply because the teams flew to LV from London; London could have been the connection from many places in western Europe outside the UK. I'm not saying that anywhere in the UK isn't possible, just that we cannot limit locales to the UK just because teams flew out from London. (As far as it goes, Wales is overdue for a leg now that recent seasons have been to both Northern Ireland and Scotland and no task has ever been filmed in Wales, especially Cardiff, which as some of us know from Torchwood
is a beautiful place, and Doctor Who
, which marked its 50th anniversary last month, is filmed there and there's both the BBC studio complex and a new Doctor Who Experience meseum there. That would be an interesting locale if TPTB paid attention to the worldwide influence of that show, as they did with Sherlock Holmes in a couple of tasks in London in an earlier TAR season.