I can't speak for how many of the UK fleet of Cessnas are fitted with a secondary seat stop, but it's something I check for when I jump into a Cessna I have not flown before. I have yet to see a Cessna which did not have some form of stop. I have seen different kinds, not always the Cessna product.
This is something which should not be overlooked. Other than in 150's, where you could reach to fly in any case, the bigger Cessnas are down right dangerous in the case of a seat release, and no secondary stop. It happened to me twice in the early days, and is very scary!
If you have to fly a Cessna which does not have the system installed, as a temporary measure, the intent is met if you reposition that aft travel stop clip assembly into a hole in the track much further forward (if it has a suitable hole - but most seem to). This will prevent the seat moving back under any circumstances. It will be a misery to get in and out, but it will be safe. I cannot speak to the local legality of this in the UK.
A good mod I have seen as an inertial reel shoulder harness assembly up under the seat, with the belt tab attached to the floor ahead. Again, I don't know how these are approved.
I hope this helps...