Obviously I'm not fully conversant in the intricacies of how each force has cut in the past, but in general most forces have done the following, or some combination thereof:

• Outsourcing services to other companies (examples include prisons, custody staff (those that work in police station cell blocks), speed camera photo analysis, HR, IT, 999 call centre staff, vehicle maintenance, other back office staff).
• Making "police staff" (non-sworn officer positions) positions redundant (in the UK you can't fire a person for (almost) any reason like you can in "at-will" employment states in the USA, you have to make the position redundant - so you can't hire someone else to fill it)
• Forcing officers to retire after 30 years service (Contractual Clause A19) - this tends to unfairly affect the specialist officers - dog section, crime scenes, traffic officers etc.
• Lowering new entrant salaries - the new government-mandated base police salary for new entrants is now £19,000/year - forces can opt to pay more than this if they choose to do so.
• Not replacing retiring officers - in some forces where an "excess" number of officers has been determined when officers retire they are not replaced.
• Where possible, transferring the jobs of "back room" police officers to non-sworn employees or contractors - this includes custody staff as mentioned above.
• Moving officers from task forces, specialist roles etc to rapid response (Patrol in the USA) - this lets the overall number of officers drop "without affecting" front-line services.
• Reducing the number and/or scope of projects - new IT systems, new cars (therefore keeping cars for longer), upgrading systems, purpose built new buildings etc
• Moving to newer/more efficient buildings to save on operating costs - some of the historical buildings are being sold off or closed because they cost too much to maintain.
• Closing police stations and police station counters - some police stations have been closed entirely, and a large number of stations that have been retained have lost their public-access front counters

Generally the police have cut about as much as they can now without affecting front-line services, but now when the next cuts come in they will have to start reducing the number of officers working the streets.

Obviously this is a very bad idea, whilst crime in the UK has been dripping, response officers are already running from job to job most nights and they can't tolerate much reduction in numbers. It's not uncommon for all response officers on a shift in some towns to be on a call or doing paperwork, and for officers to be diverted from lower-priority jobs to higher priority ones two or three times in a row before they reach anywhere. Officers are also routinely sent to calls alone with no backup being available without said backup having to come from another call or from the station. And even on more dangerous calls (fight in progress, domestics etc) it's routine for no more than two officers to be dispatched.

Now that the police are facing further cuts, some forces are planning which jobs they won't send an officer to at all - they'll resolve them over the phone.