From FODCOM 09/2010 (don't know if it's been superseded).
In the UK, for all stages of flight, instructions to climb or descend cancel any previous restrictions,
unless they are reiterated as part of that instruction. Additionally, to emphasise the UK differences from
ICAO, when an aircraft is on a SID and is required to climb directly to the cleared level, ignoring the
vertical profile of the SID, controllers will include the word ‘now’.
e.g. “Jet 123 climb now FL 120” means that the aircraft should climb directly to FL 120, ignoring the
vertical profile of SID.
When an instruction is issued that amends a SID route, the level restrictions associated with that SID
are no longer applicable. Therefore, in such circumstances, controllers will reiterate the level profile to
be followed as part of such an instruction
e.g. “fly heading 095, climb now FL 80” or “route direct EFG, climb now altitude 5,000 feet”.