Someone being 'known' to the security services does not mean that they are a risk.
The security services will know something about everyone who has ever held security clearance or who has physically visited one of their location. They know something about people who have visited countries in the Middle-East, Asia, or the Ex Soviet Union countries. They know about anybody who has held public office, especially if that office manages sensitive data. There are probably any number of demonstrations or events where they will identify the people who attended, and keep a record of the people, so know about them too. Any of you reading this could be known to them in one way or another.
Just because you are known to the security services does not mean that you are automatically a terror suspect. Sometimes people are known for good reasons. Sometimes they are known but not judged to be a risk worth pursuing. Sometimes they get it wrong. Sometimes they have not got the correct resources.
@Titus. If you were to say that one in a hundred of the people who fit a particular profile that they know about may be a risk (and I emphasise that this is a number plucked out of thin air), then that means that 99 completely innocent people may end up being monitored.
So if you (for example), were next to a group protesting about Margaret Thatcher, took part in a Student protest about fees at the Houses of Parliament, were a member of CND, went on holiday to a former Soviet country, the Middle East, East Africa or Pakistan, and regularly use TOR or encrypted Bit Torrent, you may be on their list. How does that make you feel. Good? Still want to give them the power to look at what you are doing online?