An interesting difference in philosophy between the UK and US. In the US, filing an IFR flight plan and receiving a clearance is effectively a contract between the pilot and the system. The fact that the flight moves into class G ("uncontrolled") airspace doesn't invalidate that contract. Pilots are not expected to change their mind and go somewhere else just because ATC can't tell them what to do because of airspace classification. ATC expects the pilot to follow the plan, and show up as expected even if there are periods of no radio contact (because of terrain or RT failure). Pilots expect ATC to handle their flight when they arrive. Yes, they may have to hold as directed, or take a vector, but they are never "out" of the system.