My wife and I have a friend who was Spec Ops for us in Nam. He serves as a consultant to various Swat Teams and probably to our military.
He told us that a lot of the special unit guys for London's police force are former SAS guys, both retired and guys who are out of the military.
He said that they train the London Cops on a regular basis. However, in most cases where a gun is used, the officer was a retired SAS or had served. One of their trade marks is no bad guys live after being shot by them.
Re the regular SASers being busy. He predicted that within hours after 7/7 they were flying to various parts of the world to settle accounts.
I will see him this afternoon and ask him.
Regardless, great job by these brave men. The good citizens of London can be assured when there are men like that protecting them.
I'm coming around to your point of view. Here's a security pundit arguing that same case persuasively.
http://news.bbc.co.uk/1/hi/uk/4707781.stm
There's also a link to a video interviewing the guy whose eye-witness account of the 'arrest' has been so widely publcised.