I think your research into photos is impressive, but most of them are from the centre of Bristol or from Clifton, whereas the heavy snow was reported to be in rural areas to the south of the city. Although I can't find any images from the immediate area in question, the following shows the conditions on high ground about 12 miles south of Failand on the morning of the 18th:
http://www.snow-forecast.com/photos/5638
Who can tell what's under those bumps in the snow?
It's also very debateable if Failand is infact higher than Bristol.
It's on the 145m contour, so its almost 500 feet higher than the centre of Bristol. But of more significance is the fact that the centre of large cities in southern England are rarely affected by heavy snow as they generate their own micro climate, helped by people, traffic, industry and (in the case of Bristol) by the tidal Avon. On the ridges above Bristol, snow patterns can be very different, driven by winds coming up the Bristol Channel and across the Somerset Levels.
But even if the snow in Failand on the 18th was not as heavy as the BBC and police reported, it is worth noting some of the comments made by people in the Longwood Lane area:
"The road is used as a cut-through during the week, but theres not so much traffic at the weekend."
"There are usually runners and cyclists using the lane at weekends, but there would have been fewer people around because of the snow."
"I didnt ride along the lane for about 10 days because there was permanent snow or ice ... There was a lot of snow I cant remember so much snow falling here since 1982. ... "And were quite high up here. Its windy and the snow does tend to drift."
So, perhaps very few passers by (and probably no quarry traffic) over the weekend, and those who were in Longwood Lane were doubtless concentrating on not slipping on the gradient by the quarry, rather than looking carefully at the verges.
Then, another snowfall on Monday morning, obscuring whatever might have been vaguely visible over the weekend.
I've no idea, of course, if this was the scenario, but I wouldn't discount very poor weather conditions in Failand. I happened to be at South Bristol Crematorium yesterday, which is less than 2 miles from Longwood Lane, and the weather was very much rawer than in the centre of Bristol, just down the hill.