How many police dogs are there in the uk?

It's impossible to say. German Shepherds, Belgian Malinois, Doberman Pinschers and Rottweilers are the most commonly used.
There are many different breeds, and even some 'mutts' used in law enforcement work. German and Belgian Shepherds are probably the most familiar type associated with police work, but, there are many other breeds represented. Bloodhounds - Rottweilers - Beagles - Retrievers - almost any breed that has a particularly specialized talent that is associated with that particular breed (e.g.- smell - tracking - etc.) can, and will, be utilized.