There's loads about this issue on these boards, try also in the private flying section. In the UK if your FPL takes you outside controlled airspace then at that point the centre will dump you and forget about you. It's up to you to decide who you want to work - if you know in advance the centre may hand you over (or pass a pre-note) if they have the time but there's no requirement for them to do so. You'll also see in th private flying section that many people prefer to go it alone in class G and not take a service for various reasons you'll discover. If you want to re-enter CAS it is your responsibility to get a clearance to do so from the appropriate agency, but London/Scottish Info can probably help you here, maybe fisbangwallop can confirm this?