sharpshooter41.
The policy in the UK is valid everywhere,not just the London TMA.It used to catch pilots out when arriving at our busy airport in NE Scotland,when we issued delay less than 20 mins.We did have pilots arrive with less,and in one case no holding fuel and wanting priority.That one had to declare an emergency.
I think it's becoming a standard practice at a lot of busy TMAs and it reduces controller workload.It also improves safety as accidents like the Aeromexico DC8 shouldn't happen.
