FIS is a non-radar service...although you may be asked to squawk, the controller providing the service isn't looking for conflicting traffic....he may not even have a radar..London Information certainly doesn't. Admittedly, when traffic is light, the controller may give radar derived traffic information but he has absolutely no obligation to do so. Under RIS however, the controller must pass traffic info...this means that he/she will watch your track and call any conflictions fast, slow or otherwise...this is why some units (particularly around the London area) are reluctant to provide it. In respect of fast jets, they often operate below radar cover so may not be 'seen' on radar.
This is all spelt out in the AIP, which, I think you can access via the AIS website, sorry don't know the address.