I was at Greenwich Park, and the security was very slick. We were through in minutes.
First of all there was a ticket check, and then onto an airport style bag and body check. Bags, coats, and belts were removed and went through the x-ray scanner, and then we went through the metal detecting arch. Dh had to go through twice because his belt had metal on it. Food was permitted, but not liquids, cutlery or anything sharp. The checks were done by uniformed members of the armed forces, and they were very friendly.
At Greenwich there were huge queues for food outlets, so I would recommend anyone going there to bring a picnic.
I was quite nervous about going through security for my own reasons. I had a pacemaker fitted last year, and have not flown since. The guidelines now say that it's safe to walk through metal detecting arches, but that you may set off the alarm - as it was my first time I felt quite anxious. When I showed the soldier my ID card, he looked a little nonplussed, so I was mightily pleased when I didn't set off any alarms and glad that I didn't keel over!