Just to remind you - there is an anti-terrorism hotline for UK 0800 789321.
Here is a link showing the poster that Greater Manchester Police started to distibute at MAN a couple of years ago:
http://www.ringwayreports.co.uk/anti-terror_poster.jpg
As a consequence of this, all my spotter guides for UK airports have this hotline printed on it. The spotter guide for Heathrow also have suggestion for how to behave when being approached by the police. In September I had a close contact with a threesome outside T4 at LHR, and as long as you knew your aircraft, answered their few questions in friendly tone they didn't create any problems for me.
The one page Acrobat formatted spotter guides can be downloaded from here (right column):
http://www.plane-spotter.com/Airports/index.htm
Kurt - editor PLANE-SPOTTER.com - the non-commercial aviation enthusiast website