You've got it, more or less.
In UK airspace, a UK issued PPL can fly under IFR but only in
- Class G, and
- in VMC (3000m vis and clear of cloud, basically)
An RAS is available only to IFR flights in Class G.
And yes it could take you into IMC, in which case you have to refuse the instruction if you don't have an IMC Rating or an IR.
Practically, an RAS is of little value, because of the wide separation which ATC is obliged to apply. On the very days when you may want it (lots of traffic) they send you all over the sky.
I've noticed that jet transports tend to ask for RAS when in Class G, but they don't spend much time in Class G anyway.
RAS/RIS is a UK only thing; you can't get it elsewhere. Also, in every country I know of (except the UK and maybe Australia?) you need a full IR to fly IFR regardless of actual conditions.