OK I think I'm getting there now,
Class A SVFR IMC rated 3k vis, non IMC rated then 10K vis.
Class D SVFR IMC rated 1800m vis, non IMC rated 5K vis.
So when L2K offered me a SVFR departure it was probally because the VFR minimum of 5k had lowered to something less than that, but that seems odd as I thought they didn't recognise the IMC.
Just to add a bit more confusion, recently I visited Southampton in great weather but my clearance into and out of was SVFR. On the other hand when I have transited Gatwick zone no mention of SVFR and usually I get put on a radar control service.
Thanks for the help as I quite like the SVFR flights it adds a little extra challenge for a VFR pilot.