RR,
good question and I hope you will chat to your friendly flying folk about this.
ATC is there to provide EVERYONE(B747 or c150) with the best information that they can for EVERYBODY'S safety. (its our job, we are trained to do it ,we get paid for doing it and please dont tell management but we actually quite enjoy doing it!).
When you talk to London Information you are talking to a trained senior assistant who can provide you with generic information about the airspace you are in. (London Information has no Radar display to guide them).
However when you are close to an airfield, the approach controller can give more detailed traffic information as he can see it and sometimes you on radar. So although the controller may say FIS you are really ,when necessary getting a limited RIS. This helps you to spot exactly where your traffic is, it helps the controller because otherwise he would have to take 5,000 feet or 5 miles from unknown traffic and it helps the bucket and spade brigade to get back 5 minutes earlier without jolting their G+T's.
If ever a controller sounds alittle Pi**ed off that you have called please dont take it as a personal affront, we are feeling undervalued at the moment and sound just as Pi **ed off when the big boys call as well. (see I told you its the same service for everyone!)