I would have thought that by notifying them beforehand with the 24hrs notice as required complies with the regs, and then phoning them within the 24hrs of an intended change would be perfectly reasonable imo. After all the bad guys wouldn't bother at all, so the fact you are notifying them should suffice, what if you had had to divert due to weather on route.
Interesting one this, the rules are a bit ambiguous but I've always taken the view my obligation is to give them notice not ask their permission.
The relevant bit is from Schedule 7 of the Terrorism Act 2000
Where an aircraft is employed on a journey to which this paragraph applies otherwise than to carry passengers for reward, the captain of the aircraft shall not permit it to call at or leave a port in Great Britain or Northern Ireland unless—
(a) the port is a designated port, or
(b) he gives at least 12 hours' notice in writing to a constable for the police area in which the port is situated (or, where the port is in Northern Ireland, to a member of the Royal Ulster Constabulary).