File it all IFR if the cloudbase is that low (if you are in a single, give yourself a few extra hundred feet if you can. If that doink fails or you loose a mag, you'll be glad you did).
In the flight plan, if you don't actually join CAS, you can get a Traffic service or higher (Deconfliction) from London. If you do, your first point on the FPL will be you CAS joining point. Do not get there without getting CAS joining permission - they will tell you.
You first call could reqest CAS join at XXX FL XX, IFR flight plan activation and a DCS outside CAS.
If you are remaining clear of CAS, a FPL would allow you to become expected and cut out RT chatter, but as mentioned, not required but is safer incase you need help or need to become visual at some point - or don't and have to hold / divert.
If you are planning to cloudbreak into a small field, use a procedure that gives you safety and ideally have at least a traffic service (ie - pick up a radial off of a VOR and descend to MSA). You could shoot an ILS into an airfield appraoch aids and break off VFR to land at you destination.