The liability for recall is also subject to an appropriate call-out notice being signed by the Defence Secretary and under the terms of RFA96 for a true national emergency (so not just a bit of a NEO or a UN relief mission in Libya). I've never heard that the RN and Army can 'opt out' I don't believe this is true either as terms of Service are well controlled on a tri-Service basis (but I could be wrong).
From the 2008 edition of the RFA 96 Act:
10. RFA 96 restates the existing call-out powers, but within a new framework of call-out
orders.
11. Thus Her Majesty may make an order authorising call-out:
12. The Secretary of State may make an order authorising call-out:
13. The making of an order also activates the corresponding power under the ReserveForces Act 1980 as amended. The warlike operations power has been extended for the first time to all Reserve Forces. (Under the 1980 Act, it did not apply to ratings of the Royal Naval Reserve, members of the Royal Marines Reserve or to the Royal Auxiliary Air Force).
14. There is a new power, under which the Secretary of State may authorise call-out;
15. This new power is intended to cover peacekeeping, humanitarian and disaster relief operations. The wording is specifically designed to exclude the call-out of Reservists in theevent of civil disorder in the UK. Transitional members of the Reserve Forces cannot be called out under this power.
Paras 14 & 15 would appear to be a slightly worrying 'catch-all' given the recent history of using the Armed Forces for interventions of choice rather than necessity
