"Always assume that no-one will be able to provide anything other than a FIS when you're flying under VFR "
Firstly, what use is an FIS? "Seven aircraft known in your area". Nobody has a clue where they might be. Then there are the twenty others who aren't talking to this frequency. Plus another thirty who aren't talking to anybody. I am sure that if everybody flying called up London Info, the system would collapse instantly. I never bother with talking to a non-radar FIS, though I always tune in for a listening watch, in case I have to make a mayday call.
Secondly, IMHO it's worth talking to any radar unit because they then have your verified level and they pass your details to other traffic which is IFR, perhaps in IMC.
Very different abroad though; in France one tends to get an FIS which is a sort-of watered down RIS. Plus radio contact can be more or less mandatory (Greece). So one can't generalise because some people do fly abroad....
I think RAS is only for OCAS, and nowadays is offered only to IFR flights.
Daifly - I don't doubt you for a moment but I also think that many fatal GA accident analysis is guesswork. I've read loads of AAIB reports and it's clear they often don't really know. No CVR, no FDR. Just a radar trace every X seconds. Especially structural failure cases (arguably the most disturbing to a serious IFR pilot; mid-airs are a largely statistical numbers game) - no record of preceeding control movements. A lot of it is like watching Walking with Dinosaurs and hearing how many times a day they had sex.