Hi again Priscilla,
The reason a pilot requests a service is so that both parties are completely aware what service is being provided. It is like forming a contract between the atco and the pilot.
Inside controlled airspace in the UK (class A-E) all VFRs are given a control service, wither or not radar is available.
Outside controlled airspace, Classes F & G, is where either a FIS or a RIS is given. There are a few differences between them, the main ones being that aircraft dont need to be identified for a FIS and traffic info is not given from radar derived information. What service a pilot gets depends not only on what he is looking for but also on controller workload. A RIS is obviously higher workload so can be refused.
In the UK in class E a pilot will be passed traffic information on IFRs. No avoiding action will be given to the VFR. It is up to the pilot to maintain his own seperation. VFRs dont need to tell ATC that they are in class E airspace, so it is not a totaly known environment to ATC, therefor traffic info etc can only be passed on known traffic.