At the centre of every foiled terrorist plot is a "Covert Human Intelligence Source", or agent. Recruiting these agents is absolutely central to the work of both the security and intelligence services, and both organisations are tasked by government to do exactly that. The purpose of recruiting such sources, whether at home or abroad, is to keep people safe and to prevent, investigate and disrupt terrorism. The work of these sources has undoubtedly saved many lives, often at great personal risk to the sources themselves.
The only practical way to get a human sources is to approach someone and ask, and sometimes it is necessary to ask several times. Many people would regard and approach from "spooks" unsettling, and uncomfortable, but that does not make it improper or mean that in itself it constitutes harassment. In fact, it could be said that normally, if normal is a concept which applies here, it would takes lots of attempts to persuade someone to co-operate. Even if someone is happy to work with the security of intelligence service it then can take a long time on both sides to get trust established, with pieces of information having to be checked out time and time again.
It should be said, of course, that Jeremiah Adebolajo categorically denies ever acting as an agent or covert source, and that the content of this blog is about the general issue and not his particular case.
Mi5 publish on their website information about how they operate with human intelligence sources and the legal framework for them to do so. The main legal underpinning for such work is the Regulatory of Investigatory Powers Act, or RIPA. The security and intelligence agencies don't have powers of arrest, nor can they compel someone to work for them. What constitutes harassment or implied threat in the eyes of one person, can be, to a large extent, a subjective judgement.
For instance, in a theoretical situation where a person known or believed to be associated with Islamist extremism was intercepted en route to, say, Syria where they were planning to join an al-Qaeda affiliated group it would be legitimate for them to be approached along the following lines:
The person is informed that the security service know what their plans are, and that if they were to return to the UK and continue along the same path then there would be little choice but to investigate. However, one way in which that could be prevented would be via co-operation with the service. Such an approach would hardly constitute harassment or threat in the eyes of most of the British electorate.
In cases where either agency is tasked to approach a human intelligence source, there would normally be clear reasons for doing so (which is not to imply the guilt or complicity of anyone approached) even if it was to eliminate a person from an investigation. A widespread "fishing expedition" involving trawling anyone of a particular background "just in case" would involve massive resources, more than the authorities have at their disposal.