Well, to use good examples EGLL or EGKK are about as good as they get for restrictive SIDs....fastair xxx climb and MAINTAIN 5000'. No ambiguity, the clearance supersedes the previous.
And that is where we have a problem. If we were to say that here we would have 50% of pilots asking if they can climb unrestricted or if the hold down at xxxxx applies. 25% would climb as you suggest. 24% would still follow the SID and the final 1% would bust the clearance anyway
Unfortunately it seems different countries are coming up with different procedures and solutions and this just leads to more confusion, especially for infrequent visitors to our airspace.
To my way of thinking something like "climb NOW altitude 5000 feet" might work better.