Section 10 of the Public Service Pensions Act 2013 sets two ages. Normal pension age is 60 for officers who retire from the force. Deferred pension age, for people who leave, is State Pension age (or 65 if higher). Regulation 91 of the 2015 regulations follows that: a deferred member gets their pension at State Pension age.
You can claim from 55 under regulation 96, but the reduction is measured from State Pension age. GAD's original 2015 table for deferred members (Table B) paid 0.492 of the pension at 13 years early, so someone with a State Pension age of 68 claiming at 55 would get about half. GAD has since revised the factors; ask your administrator for a current quote.
If you are 55 or over when you leave, it is different: regulation 90 lets you claim straight away with the smaller reduction for retiring members, measured from 60 (about 20% at exactly 55 under the factors in force from 1 August 2026).