From what I recall of the training I did - which was many years ago the qualifiers light, moderate and heavy were supposed to be used in the context of what should be expected in the local environment. Hence, what might be moderate snow in the UK could be considered little more than an insignificant flurry to a Scandinavian.
It's also rather subjective - each individual doing an observation has to decide on the qualifier. Remember also that the report is based on what was happening at the time of the observation - and for the preceding 10 minutes - and unless a significant change has occurred, that report will stand. Admittedly, though, I'm pretty sure the cessation of snow is classed as a significant change.
And then there's the 'automatics'. I'm not sure of how met obs are done at LHR these days but many large airports in the UK now use automatic observing stations to prepare an ob. The ob is then validated by a human (maybe someone else can tell you how much training a validating observer gets) - and that human, of course, makes a subjective assessment on which any adjustments are made. When I last worked with any of these automatic observing stations the sensing systems for precipitation were notoriously easily fooled and inaccurate.
Put all this together and maybe you can see why the ob you received did not accord with your mental picture of what was there. Or, to put it another way, it's surprising that obs are not misleading more often!