It's a one-way valve unfortunately. If you 'save' money on your project due to unusually favourable exchange rates you don't get to keep it - HM Treasury sucks it back. If your project takes a major hit at a contractual payment point then you either swallow the hit in some way or buy less - HM Treasury is unlikely to help.
HMG does not hedge money either, as a commercial company would, as that would be akin to insider dealing on the UK economy! Anyway, the Treasury does not lose if the exchange rate rises or falls as the MoD takes the hit.