You still use FIS as long as it's backed by radar rather than rejecting anything lower than RIS\RAS ?
Not as such - the options are limited. In the UK, FIS cannot be backed by radar in a manner which is in any way useful. And abroad, one doesn't normally get a choice of service when VFR - it's only FIS.
Let me explain it hopefully more clearly.
In the UK, a controller doing
FIS for you is not permitted to pass you anything based on any radar info which he may have access to. So even if he can see you head-on with another aircraft he is not permitted to warn you. (I am sure ATCOs here will correct this if wrong but I am pretty damn sure it is the procedure operated). So you cannot ask for a radar service with an FIS.
In the UK, you can ask for an
RIS and (if you get it - it is subject to controller workload) then the controller is permitted to pass you other traffic info (bearing, and level if known). He is not allowed to suggest which way you should turn to avoid it, though they sometimes volunteer this info if you say which way you are turning and it happens to be the obviously wrong way...
Abroad, there is no formal "RIS" or "radar service" (that I know of) and they just call it
FIS. They usually do have radar and usually they let you know they can see you on it. They will then pass conflicting traffic info in an informal manner, but Europe generally has so little GA traffic that this is a bit moot. You can fly 200nm in the UK and see 20 planes; 200nm across France or Germany you might see two; 200nm across Italy etc you are unlikely to see even one. (Admittedly this may be because abroad one can easily fly a bit higher, say 8000ft, whereas UK airspace keeps one lower where most GA pilots fly anyway). So, abroad, you just sit there on autopilot in relative peace, eating up the miles while taking pictures and making movies
The UK system where FIS does not provide radar based info is to do with the watertight separation between the IFR enroute sector control (London Control etc) which does airways traffic, and all the "VFR dross" below. If you are VFR and call up London Control for a service they will politely tell you where to go. Abroad, they don't have this separation and you can fly e.g. across Belgium at FL80 VFR while getting a radar service, possibly with vectors which is really nice and easy, from say Brussels Approach. Much more pragmatic.
In the UK, to provide some kind of radar service to VFR pilots, the LARS system exists though the main purpose of it is to support the military (who historically did not have decent nav capability and used the map/stopwatch method and were frequently getting lost, and anyway have awful fuel endurance), and also you can get a radar service from airports e.g. Southampton or Thames Radar but they don't really like doing it.