niknak,
Spitoon has given the reference -
c) in the absence of any ATC directions or regional air agreements, operate the transponder on Mode A Code 2000
As far as I am aware, Norway, Denmark, Holland, Belgium, France and Ireland all specify 7000 as a code to be used by flights where no other code is assigned (conspicuity code).
I believe that it is part of the European regional air agreement.
Thus the only time that an aircraft entering a UK FIR will have 2000 set is when they are leaving one of the Oceanic FIRs.
Do you think that an uncontrolled VFR flight from say France who has probably had 7000 set since departure should set 2000 at the FIR boundary and then (in order to comply with the UK rules set 7000 again after some time period?
regards,
DFC