ontact your local force. However the chances of you getting a companion horse, let alone one that can be ridden are virtualy nill. Sometimes yu get ex police horses that are ex because they have serious issues.
They can often work until quite a good age. i know 1 who is 23 and still pounding the beat. ILPH sometimes gets them. sometime they fail to make the grade for quite trivial reasons (trivial to us) but they have gone through all the training. They would be quite a good buy.
If they don't make the grade sometimes they go back to where they came from. The dealer that iused to work for supplied a couple of forces and we got a couple back. So if you know who supplies them, you could put your name down with them too.