Before you get too far with the impression that they are some form of secret body ...
The Forward Intelligence Teams are part of the Public Order Command structure. The Met originally, but most forces now, have a central unit tasked with planning for known or predictable public demonstrations / disorder. These events include ceremonials, football and other sports events, demonstrations, protests and everything else likely to require resources other than a local police presence.
In order to facilitate lawful protest and minimise criminal activity within otherwise lawful protests (and there is a long history of criminals hiding within otherwise lawful groups in all sorts of contexts) the police have moved towards an "intelligence-led" approach, with a view to ensuring that higher levels of policing are directed only at times / places / groups known to present a higher threat of unlawful action (the flip side being that times / places / groups not known to present a higher threat receive a lower level of policing, such as is seen in many situations every day.
The FIT teams are a necessary part of that intelligence-led approach. Where planning meetings are held by protestors then the police will use the lawful means available to them to gather intelligence, including trying to identify those involved (as there is a very strong correlation between serious unlawful protest / other activity and those with previous convictions for such offences) and to work out what is going to happen on the day(s). Intelligence is also gathered from open sources such as posters, leaflets, papers, media coverage and, er, the internet (* Waves to CO11 Public Order Intelligence Unit *).
They are not "supercops". They are ordinary officers seconded to the units from other duties. They have no special powers - they are bound by the restrictions of RIPA, etc. They are not paid any differently from other officers of similar rank, grade and service.
Anyone who considers that the activities of the FIT team officers is excessive or unlawful is able to complain just as anyone can about any individual officer. Likewise if the person believes they have been unlawfully targetted by the organisation. The civil law is available to anyone who claims to have suffered harm as a result of their activity.