Hi Dick
I operate VFR / IFR regularly outside CAS. Not being required to file IFR outside CAS and Off-routes gives you a lot of flexibility to decide "I don't like the weather" then climb and go IFR. Clearly, there is an exposure to bumping into someone doing the same thing and, whilst you're not required to talk to anyone, i certainly get a radar service wherever one is available (and tell them that i'm IMC) - simple common sense.
There are no charges levied for using a Lower Airspace Radar Service in UK. (You only start hitting charges when you start doing approaches). As for places being closed at weekends - more prevalent to the military establishments, but since LARS has started to be offered by a greater number of civvy providers there is more availability at weekends.
Specific to the Plymouth area is a radar service called "Plymouth Mil" which exists to support the extensive Naval training areas in that area and offers a LARS as a secondary function. It closes for significant periods over the holidays and is therefore sporadic in it's availability.
In general, i've always found airfields and ATC providers to be pretty relaxed about the VFR / IFR side of things - just tell them what you're doing. The only time it becomes an issue is if you want an Instrument approach, in which case they need to fit you in with their traffic, but they can usually work something out.