Referencing aterpster's chart.
If ATC says: "Climb FL80" somewhere en route to DET. Do I climb to FL80 right away? Or stay at 6000 until passing DET (or cleared off the SID).
I know it's a responsibility to be well versed with local procedures there is no way we read AIP, AIP Supp, AIC before a given a flight to a state we haven't been for a while. Relevant stuff should be in our briefing pack, on the Jepp. plates (or equivalent). And even then there is no guarantee you get it right when your brain is set to a certain behaviour. Hence the virtue of standardization.
I go into LHR quite a bit and in practice it often isn't a problem. Your desire to not f@ck up and the known trickiness of the com procedures puts you on guard. Thus you ask when something is ambigious.