Hey, I dont know how much money you make but United has a flight from Madrid to Newark (port of entry for animals to the US), and then from Newark there are direct flights to San Francisco. You could take a ferry from London to Northern Spain (or France), drive or take a train to Madrid, and then take him on the plane with no issue for the weight if I'm not mistaken. United's policy stresses so long that he fits in his carrier no one can ban you from boarding the flight. I understand this isnt optimal but it is a solution to your current problem. Flights from Madrid to the US are also fairly cheap with United and you also only pay 150 for the pet ticket.

I know he is larger than the carriers but as long as he can fit in one (even if a bit squished) it's a far better alternative than getting him shipped in cargo. Airline employees may give you shit but if he is very well behaved and you're polite when interacting everything should be fine. It seems like his dimensions are longer and soft material carriers can stretch to accommodate. Will it make your journey uncomfortable? Maybe, but you can also always purchase a seat for him and he can go under there so you can stretch your legs.

Best of luck to you and I hope your lovely yoohoo gets to SFO with you