The note I refered to in London SIDs tends to be generic "DO NOT Climb above SID levels until instructed by ATC". This note can be found on all London Aerad SID plates.
More specifically, each SID has a warning box in the plan view stating for example "Due to interaction with other routes, do not climb above 5000 unless cleared by ATC". In this case, 5000' is the blocked/initial cleared altitude. Other SIDs with an initial cleared altitude of 6000' for example have the same wording but with 6000 replacing 5000.
However, the SIDs do not use the words "initial cleared altitude" as this could be ambiguous. They are specific and reiterate the initial cleared altitude.
In my initial post, I should have spent more time highlighting the warning box which is specific to each SID and not simply stating the generic note. Thanks TopBunk.