You can't transfer, you have to start from scratch.
I looked into joining the cops years ago when I was due to leave the mob in the January after completing my 22 years, and I started my application the previous June.
First apply to the force (sorry service) where you want to serve. (I chose Essex)
Send off the application,
Invited to sit the entrance exam...painfully simple. passed
Invited to take fitness test...piss easy...finish with the Bleep test. passed
Invited to sit the interview...lots of questions on what would you do if....
Tell me about how you have diffused or dealt with a potentially difficult situation....
Takes about 30 mins and you have 3 people on the panel, on mine was a male inspector, a female PC and a woman from the HR dept.
Go for a coffee and then go back into the room for debrief and result...Passed.
Given date for medical in the January and a rough idea of when my training would start. I was told probably Feb, but might be in April depending on how much funding the service had in the training kitty.
From sending off the application to getting through the interview was 4 months, they did have a slight relaxation on the medical aspect for being ex forces, but a Doc would still have to check the paperwork and the cough and drop.
And then Pusser said "Want 2OE?"...hmmm £40k+ as a Chief Diver..or £20k as a baby PC.... £40k+ it is then!!!
The Essex Police were pretty cool about still being in the forces whilst I applied, they get shed loads of ex services applying, so no drama for them.
My oppos son has applied for the Kent Police, it's about a 10 month process, but ex forces can be a bit quicker as there is less work, ie medical, vetting and your 264's, referees etc.