Being biased here - having been in uniform during my whole service (although I was promoted) - being a detective is not a promotion, it's a sideways move to another department ie CID, so you would become a detective constable (or whatever rank you were in uniform). It's a popular misconception that being moved to the CID is a promotion. It's just a more specialised department, like traffic, or scenes of crime.
As Ethel says, you will need to join the police and be a probationer.
This initial phase will last 2 years - once you have passed this probationary period you can then set about becoming a detective but its not guaranteed. You will then have to face competition to get onto the Initial Crime Investigators Programme (ICIDP) which will be the starting point to get into a detective role. Google for Centrex (Central Police Learning and development authority) to get more info