A friend's son, whom I've not seen for some time, wanted to be a forensic scientist with the police. On a careers visit to the labs he was advised to apply for the force simply to get some police experience before he launched into his forensics career. It wasn't compulsory, though.
He applied to the force at eighteen and got in on a deferred entry whilst he went away to study for his degree. He went into the force with a first class honours in the science he needed, only intending to stay on the beat for a couple of years. In the end he didn't take the forensic route, because he was enjoying the work so much.
From what it says on
here, it looks as though you do need to do that degree, after which there is a range of places you can apply to for a training post.