Found a solution in case anyone finds a landline call being held by the AA callback despite hanging up the phone.
In the UK, only the call initiator, in this case the AA, can release the call. This is a problem if you use the AA callback from a landline to call, say, a recorded announcement or have a no reply call and hang up. The call will be held and you will continue to be charged.
The solution is to press the recall button, dial 3, then hang up.
The phone will ring six times, after which the incoming AA call will be dropped.