You are not alone - many "security" employees on the railways make the same mistake.
Quite........but let's turn the discussion around for a minute.
1. In the UK, airports and railway stations are at a "Severe Alert State", meaning that a terrorist attack is highly likely.
2. Anti-terror laws have been introduced to make an effort to disrupt reconnaissance activities by terrorist groups.
3. Most airport/railway workers, or for that matter police officers, cannot recognise the difference between a "professional photographer", a "criminal photographer" or an "hobby photographer". However, you can hardly blame anyone for that - how can you tell the difference between an aeroplane enthusiast, a terrorist or a spy? - a sad fact that has got more than a few young lads in trouble overseas on a number of occasions!
So, to exercise fairness, and ensure you can take your aeroplane pictures without hindrance, the BAA have introduced a permit system - indeed, if you were to check with the intrepid group of aviation enthusiasts who brave the elements at the airport perimeter every weekend, they are delighted to fully co-operate with airport staff and police in watching for any suspicious vehicles, activities or bogus plane spotters!
I can understand the BAA wanting anyone with tripods and stuff to have Public Liability cover - if someone trips over your kit, on those hard hard floors in Terminal 5, there could be a nasty injury for which the BAA would not want to be held responsible - and why should they be? In these days of "where there's blame, there's a claim", the PIL requirement is as much for your protection as for theirs! I am also pretty sure that our of our band of aeroplane enthusiasts, very few probably have PIL.