Anotherthing has summed it up. Only in the first call to a radar unit is the passing level/alt req'd. (unless requested etc)
I don't remember ever being asked for a passing level check by Manchester after handover from Durham App radar, or by London 134.125 from LF app, or by London from Exeter App. The list goes on. If they had asked for one they would have got one. In practice there seems to be no requirement for it.
There is no mention of 'En-Route ATS Unit' in the glossary of the latest CAP413, so we have to assume that this means any ATS unit used by the A/C while en-route.
If the first radar unit is area control ie London, Scottish or Manchester then they should receive the SSR check but not if it has already been given to the dep airport app radar.
Some pilots give passing level on handover from one area control to another. This wastes airtime and must complicate the call for the ATCO. There are too many things that have to be said on the R/T in the UK as it is.
