ATC does not want to take the risk of vectoring you into ash (even though the weather radars might provide some info on where that ash might be)?
That's what's the case here. Under ICAO rules ("recommendations"), ATC may not and will not vector you, or offer you an IFR clearance, into areas that may be affected by VA.
Weather radar detects moisture, not VA. So weather radar (either ground-based or on-board aircraft) is useless in detecting the ash plume. The only way anybody can know where the ash is, is the rather coarse prediction of the VAAC London. That, plus actual observations/measurements by aircraft in-flight.
Since the amount of proper environmental monitoring aircraft is rather low for such a vast area, the various CAAs and Met offices have now started to solicit input in the form of Pireps from pilots who have flown. With the current flight restrictions, that comes down to us GA.
If this airspace is closed to IFR, it means we must fly VFR or fly IFR OCAS
IFR OCAS is pretty much a UK anomaly. I doubt whether anyone knows the exact ruleset that applies in this rather unique case.