If you depart Great Britain for one of the places the act applies to (NI, RoI, IoM, CI) then you must either:
a) leave from a designated airport
or
b) give the police 12 hours notice
The same provisions apply for arrivals.
The
Terrorism Act 2000 Schedule 7 specifies
notice not
permission.
Note that the police have the right to stop, question and detain passengers and crew on any flight.