In the UK, the following applies:
In the UK, for all stages of flight, instructions to climb and descend cancel any previous restrictions, unless the latter are reiterated as part of the instruction. However, to emphasise the UK variation from ICAO Doc 4444, when an aircraft on a SID is required to climb directly to the cleared level, without reference to the vertical profile of the SID, controllers are to include the word ‘now’.
e.g. “Jet 347 climb now FL120”
As detailed above, the use of the RT term 'climb now' shall be constrained to those occasions when the controllers require the aircraft to climb directly to a level above the SID profile, without reference to vertical restrictions published as part of the SID. Therefore, controllers must be aware that the SID vertical profile provides separation from other procedures, terrain, and ensures containment within controlled airspace. Therefore, and in accordance with legacy CAP 493 procedures, the controller continues to have the option of either:
a) leaving the aircraft on the SID vertical profile without further climb instruction (see paragraph 2.3 above); or,
b) providing a climb instruction above the SID profile and reiterating restrictions as necessary to ensure aircraft and terrain separation, or airspace containment, e.g. “climb now F120, cross XYZ 5,000 ft or above”; or,
c) ensuring aircraft and terrain separation or airspace containment through lateral instructions (see paragraph 3.2 below).
Full details in the instruction here:
Manual of ATS Supplementary Instruction 2010/04