UK Police Monitoring 24 'Major Conspiracies'

Says the Telegraph:

John Reid, the Home Secretary, confirmed yesterday that police and security services were aware of about 24 "major conspiracies". They are believed to be "multi-handed" terrorist plots, such as the alleged plan to blow up transatlantic airliners that led to the arrest of 24 people last Thursday.
Sources say that, on top of the inquiries confirmed by Mr Reid, up to 50 more are being conducted by anti-terrorist police, most of them involving Scotland Yard and MI5. Some relate to fund-raising activity... and intelligence-gathering, such as details of potential targets.

One thing about major terrorist operations is that as the size of the operation increases, it takes an increasing number of people and amount of resources to pull it off. Which means the vulnerabilities increase, too. There are potentially more loose strings that can be pulled.

Early reports say that the first tip in the London airplane bomb plot came from a single concerned citizen in the UK Muslim community. Since this was a big operation, that one tip generated a lot of payoff.