C2L27L
I'm left with an uneasy feeling about this one.
As pilots we have to be concerned about terrain clearance, and we satisfy ourselves by either being on a published procedure or route (e.g. IAP, SID, STAR, airway) or by accepting a radar vector which we trust will only be issued with radar contact and when our altitude is above your MVA.
A missed approach or a SID is a three-dimensional procedure. We're safe at or above the prescribed altitudes on the route: below the profile we're at risk, and terrain clearance is not assured.
To issue an altitude restriction that keeps an aircraft below the published altitude seems to be a mix-and-match situation. Who's responsible for terrain separation, and for how long? In the situations you describe, are you promising terrain separation at the inetrmediate altitude for the whole missed approach or SID?
At Heathrow it might not seem like an issue -- you say "No obstacles to limit the climb" but that's not how the game is played. The pilot doesn't know that, and can't assume that. SIDs aren't annotated with "this altitude is for ATC" and "this one is for terrain, you die below this".
The AA965 Cali crash (B757 vs Colombian mountain) had as a strong contributory factor a willingness on the part of the crew to accept what they believed to be a direct clearance in a non-radar environment to somewhere they should never have been going to direct, where the only safe route was the published one at the published altitude. They probably acquired that unquestioning attitude in the flatlands of the eastern US.
I can't find anything in PANS-OPS or PANS-RAC to back me up on this and perhaps it all goes out of the window in a radar environment. But I think you should issue explicit instructions for lateral guidance (e.g. a radar heading or an instruction to turn). That will keep pedants like me happy, and it just might keep the crews of your customers alive when they mishear a 'stop the climb' instruction coming out of Geneva on the return leg.