VM,
I think you're getting some confusing replies here where folks are quoting various ATC separation rules and probably not giving you the answer you are looking for.
For UK airspace the UK AIP is the horses mouth for the answer to this sort of question and at ENR 1-1-3-1 Para 2.1 it says.....
Vacating (Leaving) Levels
When pilots are instructed to report leaving a level, they should advise ATC that they have left an assigned level only when the aircraft's altimeter indicates that the aircraft has actually departed from that level and is maintaining a positive rate of climb or descent in accordance with published procedures.
HTH,
WF.