Wouldn't it be more prudent to have all 6,000 armed? Preferably with submachine guns. However they probably can't scrape up more 3,000 police who are qualified to be armed, or even that know which end of the tube the bullets come out of. :)
OTOH, the 3,000 armed police they do have might be more effectively employed storming a few mosques, particularly the one that seems to have been the common factor in all these bombings and attempted bombings.
Most of them don't have proper firearms training and hence are more likely to be a danger to themselves and the public if armed.
The Met Police firearms unit, who are trained, generally seem to hit whatever they're shooting at.
Unfortunately what they're shooting at is not always what they're supposed to be shooting at and they were already subject to various court actions even prior to shooting Mr Menezes for shooting people who weren't armed, were the wrong people and so on.
So heaven knows what the untrained ones would be like when let loose with firearms. I'd prefer to avoid finding out.