Smokie,
But you're being rostered for a combined SBY/FDP. Whilst the regulations can't cover every eventuality your SBY will not actually come to an end because it goes straight into an FDP in the same duty period and the rest period will not commence until completion of this entire duty.
To play it another way, they not roster you for the FDP, they just call you out on the SBY for it, the end result is the same but that doesn't help you plan your life.
I believe you'll find your quoted 12 hours rest is incorrect also, unless CAP371 has changed since I worked with it, 12 hours is the minimum rest at base, if you're away from base and/or in accommodation provided by the company then minimum rest is 11 hours subject to the travelling time between the airport and the accommodation.