I agree is should be behind the tray table arms, but even if not if you pulled on the strap, or moved your table it would only tighten the strap. The child is wearing a harness that is looped through the strap so tightening the strap will not tighten the child's harness. The only real danger is if you opened your table and the strap got pulled down, the child could get pulled lower in there seat, but from the picture it would have to move quite a lot before it would pull the child downward.

http://kidsflysafe.com/
On the website it does show the strap should be fitted under the table.

I don't think this strap should be used on a BA flight as BA fly under CAA rules and I can't see it saying it is anywhere. Under FAA rules under 2's don't even get a lapbelt they are just supposed to be held for takeoff and landing.


Edit: might be allowed on CAA flights but needs aircrew training see here but as Prospero's pic shows it was not fitted correctly.