Closing the other thread seemed unnecessarily over-cautious. There was an interesting underlying question (with the Cold War basing of UKAD assets, how well defended is London?) although the poster's (perhaps over specific) stated question (asking for reaction times) may have been naive (though who can't work it out?). There was no sign of anyone being willing or ready to give away any information which could remotely be considered as secret.
Besides which, I doubt that OBL would repeat his NY tactics in London (not only because his potential targets are less readily identifiable from the air, and less easy to hit than the Pentagon, set in its grounds, or tall towers). Shouldn't be worried about something nasty left in a suitcase in Leicester Square Underground? Smallpox, perhaps?