trot
as previously pointed out in the UK any new clearance cancels the old... which to me makes complete sense... therefore if you are deaparting on a SID, with altitude restrictions and I want you to climb straight away to a FL above the final SID level then I should just say (and should only need to say) "Climb FL..."
However as the CAA messed up, you will hear people use 'Climb Now" in an attempt to stop being quizzed by pilots if we mean 'now' or if we mean once the SID profile has been flown. You will also hear the ridiculous (but I've used it myself) 'Climb now, unrestricted, FL...". Unfortunately needs must and most London ATCOs will tailor the phrase for the audience... UK airline and UK sounding crew... 'climb' or maybe 'climb now'. Foreign crews often get the full ridiculous phrase in an attempt to stop ambiguity.
The CAA did try to correct their instruction, but unfortunately by then, the damage had been done.
As Pontious states, it is silly that differences should be filed for such basic things. I'm all for filing a difference... a couple filed by NATS make complete sense in a safety case, but some things are best left alone.
Departure is busy time for both pilots and ATC which does not need the to and fro of instructions being quizzed.