DaveReidUK
Byelaw 4.6 says: No person shall fail to comply with any Sign prohibiting or restricting access to any building, road or any part of the Airport. Not paying your bill as described on the sign for accessing the terminal drop off area sounds like it is covered by this. I don't expect that the Level 3 fine (£1000) will be applied because of reasonableness as noted above but on the face of it, they can set a fine and from experience with what TFL do,, the fine will be in the £120 range, reducing to £60 if you pay on first notice. If you haven't registered for Autopay, there is unlikely to be an option to pay when you get the letter letting you know that you didn't see the sign and read the small print on it.
Shall we come back to this thread in 3 months time and see what has happened? £5 says you are being overly optimistic, Dave