An employer does not have to allow a Reservist to accrue leave whilst mobilised. Some employers do choose to allow this, but you are entitled to grant leave on a pro rata basis from the point in the year when your Reservist returns.
Reservists accrue leave with the MOD whilst they are in full time service. When they demobilise, Reservists will get a period of post-operational leave (POL) during this period they will continue to be paid by the MOD. POL is accrued based on the number of days spent in operational theatre, at the rate of around 2.5 days per month.