Every police force has it's own entry criteria but entry is always at Constable level. (The government has put forward plans for entry at higher levels but, as far as I know these aren't in place yet).
Promotion is based upon experience & performance. (There are also examinations at Sergeant & Inspector levels).
Most forces have 'fast track' schemes: Entry is still at Constable level but, subject to satisfactory performance, promotions come much faster. These schemes used to be solely for people with university degrees but most forces now accept people with senior management experience as well.
Most forces used to require 5 O-levels (now replaced by GCSE's at grades A to C) including Maths & English. Formal qualifications are now considered less important - the police simply look for people who are 'right for the job' - but it's probably still a 'plus point' if you can meet the old criteria.
In short, there's no way to become a DCI without starting out as a Constable (which, inevitably, means getting punched, spat at and vomited on by hoards of drunks on a Saturday night, amongst all of the other joys of being a copper!).
While I can't find precise figures, it's worth noting that it's probably well under 1% of the police force who make it to DCI level. (Also, despite what I've written about no formal qualifications being required, every copper I've ever known at DCI level or higher has had a university degree).
Full information about police recruitment will be on your local police force's web site - and if you can't find that you'll never make a detective!
Chris