My understanding is that you do not report passing Altitudes/Flight levels, unless requested.
My understanding was the other way round - you should always give level information on first call on a new freq.
Seems to be borne out by this bit in the last few pages of my old home copy of Aerad Europe & Mid East Supplement, Air Traffic Control, United Kingdom, para 8.1.2
When changing frequency between any London or Manchester Sector pilots are required to give callsign and Flight Level (when not in level flight, the level through which the aircraft is passing and the level to which cleared)
"required" ...
Hope that helps, 411A and GS-Alpha.
Cheers