It may also be worth asking your local police force about one of their retired dogs.
I've got a former West Mids Springer Spaniel who was retired at the age of seven and who is by far and away the best dog I've ever owned! I know that West Midlands at least operate a scheme very similar to the one you describe, for the dogs that for one reason or another aren't suitable for police use.
Possibly another avenue to explore?