Thanks FFF.....
If you are flying S+L and want to climb above a layer of cloud ( through it ) try filling an airbourne flight plan. As in " G-CLUD request tempory change IFR climbing FL50 once estabilshed request change VFR on top" or something like tht, it has always worked for me...
I wouldn't bother with any of that outside CAS. I would just tell them "G-CLUD climbing FL050 IFR IMC" (if in IMC). If in CAS I'd request "G-CLUD request climb FL050 IFR IMC".
"VFR On top" is not really applicable in the UK. As mentioned, in the USA you can be issed a "IFR to VFR on top" clearance which is an IFR clearance until you cancel IFR when you are VFR "on top" again. Very useful it is too, but once ontop you are VFR again and VFR rules apply.
In controlled airspace it is a different matter of course (class Delta for IMCr holder). Here you need a clearance but they will only clear you to some point on the edge of their CAS, once you leave CAS it is up to you what you do. Inside their CAS you will be under a "Radar Control" service. ATC units outside CAS cannot issue a clearance to someone departing IFR with an IMCr (i.e. not entering the airways).