Mr Optimistic......my apologies, I misread your post and thourght as you wanted the CC to demonstrate the brace position you hadn't read the wealth of info on this.
In training we had to go through the emergency drills which include the pax brace position in our sim at Luton. I can assure you it is do able although not comfortable.
Furthermore the 319/320 seats will bend forward very easily believe me!! You don't need an impact to test that one on a turnaround and some of our pax will demonstrate it inflight to achieve extra legroom as well
Your relatives choose Thomson over EZY for this reason alone as indicated in your OP?? Have to say I'm surprised given the seat pitch for Thomson shorthaul is less than EZY.
And yes they can purchase extra legroom......on both carriers.