Hi, I can't help you directly with your query because my experience of travelling with a disabled companion is with my nearly 90 year old Mum but if you haven't come across it, you might be more likely to be able to connect with other parents who've had to find a solution to this problem here:

https://community.scope.org.uk

It isn't the most user friendly discussion forum but you'll find the users very friendly and helpful plus it gives you access to Scope's own advisory team. I tend to use the search facility which does work well rather than just browse it. In order to post to the online community you have to register but you can access the posts and read the discussions without having to do that.

Britax used to manufacture seats which could be installed in the car using only a lap belt and they are compatible with airline seats. My kids are now beyond that stage so I don't know whether any of their current seats are able to be used with a lap belt. I have always found their customer services to be helpful and it might be worth contacting them for their advice; you won't be the first family to face this issue. Also check with Thomson as to the width of the seat on the plane so that you have a narrow enough car seat.