I must be pretty sad to have registered just to post this*, but as a child in the mid/late 70s my grandfather took me to see a model that matches that description at the old Swann and Edgar department store at Piccadilly Circus.

I remember that it had a Concorde on a wire that took off and flew through some sort of backdrop.

Might be the one that GrahamSimmons saw as well?

(*I’ve lurked on here for ages, but with no background in aviation, I’ve had no reason to register before.)