As far as I can make out there are a few reasons for keeping SVFR but all of them are a cop out. Namely the requirement that ac can't fly VFR at night, which means that you have to invent something that allows them to fly visually, namely SVFR. Why don't you just keep it simple and say they can fly VFR at night and the whole problem is sorted. This is especially pertanent when the PPL has an add on for visual flying at night but doesn't go as far as requireing an instrument rating. This means that the CAA accepts the fact that IFR is mandatory at night but doesn't go as far as regulating that a pilot has to have any idea how to fly an IFR approach. Seems stupid? yes it does.
The other is that helicopter routes in Class A could not work if they were IFR so the cop out of SVFR is introduced. As someone said earlier, if you are that concerned with separartion and rules, just make it Class C airspace and all confusion is removed.
The problem within UK airspace will always remain whilst we cherry pick from the ICAO buffet. The rest of the world manages to sort these problems out whilst opperating within a known set of regulations. Whilst we in the UK bodge things together, confuse the hell out of everyone and then pat ourselves on the back for being flexible. SVFR is one example the other is ATSOCAS.