At this time they'll have the armed response units out with HK MP5s PSG1s and G36Ks, plus shotguns, handguns and various other SWAT-type gear
The Anti-Terrorist Squad (formed to deal with the IRA) and Special Branch (ditto) will be active and they are always armed. Most likely, they'll be focussed on crime scene stuff for the moment, unless they've already got a clear idea who.
Once Blair got back to Downing Street he'd most likely have called a meeting of the COBRA committe, which includes his key ministers, the heads of the military, external Secret Intelligence Service and the internal Security Services, plus the Director SAS.
The SAS ready unit in London will already be active, under control of the police. A bunch of their pals will also be on their way from Hereford with suspicously heavy kitbags if contact with the perpetrators is contemplated any time in the near future.
When they are found, assuming they aren't already dead in the explosions, if they resist capture the police will hand control to the SAS.
Then they will all die.
Please let them resist capture.
Don't forget this guy too: