I travelled to Europe when my son was an infant and had huge trouble finding a restraint for him which anchored to the car as well as using the seatbelt - all the available restraints were only held in with the seatbelt. Ultimately, I bought a seat brand new on US ebay which had the capability to anchor to the the vehicle and had it shipped to my dad's house in Spain. His car (a BMW) had the point for an anchor, but no bolt attached so I took one over with me.

My point is that your DH is probably correct - the seats in the UK may well not anchor to the vehicle, they may just be held in by the seatbelt! As someone else said, fitting a car seat is not difficult... perhaps if you practice a few times (get someone who knows what they're doing to explain it to you) you will be able to do it at the airport yourself. As for the UK, check whether the vehicle you will be putting the restraint is has a bolt or at least the anchor point so you can take your own bolt if required.