SASless, you misunderstand the UK's concept of 'reserves'. If someone joins the regular armed forces, subsequently decides that it's not for them and decides to leave before their period of regular service has been completed (referred to as 'Premature Voluntary Release'), then they are 'transferred to the reserve' for the balance of their remaining time.
This means becoming a member of the 'RAF Reserve of Airmen' (or Officers, as appropriate). For this they receive no identity card, are paid nothing, receive no training and cannot even use the facilities on their local base.
In the RAF we also have the 'Auxiliaries'. These are part-time volunteers who man certain RAuxAF squadrons. They have a training commitment and are paid whilst on duty. They have identity cards and can use the base facilities. There are no Reserve Air Force flying units, we have no Air National Guard and no Royal Auxiliary Air Force flying units......
Under 'Total Force Concept', we have a certain number of regulars. These can be augmented by the auxiliaries fairly easily. But when the Secretary of State for Defence enacts an Order under the relevant Section of the Reserve Forces Act, many of those who have been 'out' for years will receive a call-out notice giving them a time and date at which to report to a training centre before being sent to the appropriate unit.
With the 'contractorisation' of certain trades, it is increasingly likely that 'call-out' of reservists would be required in time of conflict. A money-saving defence-cutting idea to reduce the number of regulars - but these civilian contractors can't be ordered to do anything which isn't in their contract.