Chiming in as I am making the same plans for my cat. I posted the same question on a longer thread about moving pets, but will add some comments re: flying with pet in cabin. For me, I do think in cabin can be good (maybe more for me than the cat), but I took mine on a 20+ hour flight from the US to China and my cat only meowed at the very end when we landed in China. She was a pro the rest of the time and I think appreciated being petted by me from time to time. Plus, I could give her a little water here and there to make sure she stayed hydrated since she was stuck in her carrier for so long.
I think the biggest concern for me is clearing customs in the European city you land in. For me, this will be Amsterdam, and we will switch to cargo from there. Now I am being told we must clear customs there, but no one seems to know if my paperwork for UK customs will do, or if I need something else. It seems like the requirements are the same, but hate to make a mistake there and have kitty get in trouble during her transfer to our second flight. We have to stay overnight too because there are only certain "animal-approved flights." Definitely check in with the ARC in the city you'll land in in the UK if you go the cargo route for the final leg of your journey. I have also read you can take the Eurostar or certain ferries, but the cat must be in a car at all times. That wasn't very convenient for us, but sounds like it might be an option for you and probably a lot cheaper than flying your cat in cargo.