On the en-route radar side of ATC in the UK.
When being transferred to a London ATC radar control service from another radar control service be it inside or outside the UK FIR then all we need reporting is the cleared or cruising flight level.
If you are transferred to London from any other service (ie still outside CAS) then you will need to be verified as well as validated, therefore the passing FL will also be needed on first contact. The only exception I know to this rule is when transferred from many of the military units to London radar as the military units are deemed capable of validating and verifying Mode A/C.
Please acknowledge the instruction to Squalk Ident when requested even if its just a 'roger'. If we know you have acknowledged then we can be confident the ident we see is you and then the ident has been successful.