So, my question... How is it that you can have an IFR ticket in the UK but not be qualified to fly in IMC? This may just be something I've missed, in that I don't have an IFR ticket, but I've been in the business on the ATC side for 26 years and it's never occurred to me that someone licensed for IFR would not be able to fly in IMC.
Couple of differences to appreciate as background.
* The UK does not permit VFR at night. All flight at night has to be IFR or SVFR (in a control zone).
* The UK has very little class E airspace -- the vast bulk of low level airspace is A or G. IFR in class G is standard practice, is the only way of going through class G at night, and is based on (as rodan quoted) the quadrantal rule for keeping aircraft apart from each other and the minimum height rule for keeping aircraft apart from terrain and obstacles. Radar information and advisory services are available to reduce the residual risk.
With that in mind, it makes more sense to set licence privilege restrictions outside controlled airspace in terms of weather minima, not flight rules. Thus we don't have "IFR tickets" as such, at least for flight outside controlled airspace. A pilot without an instrument ticket of any sort is limited to a minimum flight visibility of 3 km.
...that RAS required the aircraft to be on in instrument flight plan. Isn't a pilot on an instrument flight plan, by definition, capable of flight into IMC?
The idea of an "instrument flight plan" is also alien to us for operations outside controlled airspace, at least in the way you think of it. Flights outside controlled airspace do not require a clearance to fly IFR, so they can switch at will between VFR and IFR (provided they obey one or the other). While a FPL can be submitted for flights outside controlled airspace, the initial choice of flight rules is not limiting.
All that said, except at night, it would be unusual for a flight operated visually by a pilot without an instrument ticket to claim to be IFR simply to receive a RAS.
Within controlled airspace, the UK is much more normal (if there is a "normal"). An instrument ticket is required for IFR, a FPL and ATC clearance is required, and IFR flights are separated by ATC.
Finally, worth adding that the RAS-only-for-IFR rule was introduced maybe 10 years ago. Previously, a RAS could be given to VFR flights -- IMHO that made a lot more sense: if you can't sight the traffic, you may need avoiding action, whether you're VFR or IFR.