Current financial regs allow you to be paid up to around 200 pounds per day while mobilised as long as you provide the necessary documents at Chilwell. This replaces the old Reservist's Standard and Hardship Allowances that many suffered under. Medics can get more but I assume you're not one.
If you earn more than that then you can either accept the pay cut or appeal - or get your employer to appeal. As to whether or not you'd be successful, your guess is as good as mine.
I'm pretty sure that since you're working abroad you're exempt from mobilisation (the same would apply if you were an oil rig worker or merchant seaman). You have notified them of your location I presume?
I'm not 100% sure about this, but I'm pretty sure that's what it says in my green book.