The regs that state there must be 11 hours between duties, and that a shift can only be changed to a rest day with authority of the chief constable, and then there has to be good reason (such as an unforeseen event). If it is changed you are entitled to payment at an enhanced rate depending in the notice period.
If there is no good reason, or exigency of duty as they call it they may not change your rest days. Having insufficient staff on the unit by choice is not a good reason in my opinion.