Mjd, to add to what everyone has already said, you can note that the current set of Ordnance Survey 1:50,000 maps of the UK treat A-roads in exactly the way that morwen (and others) have said. If they've got green signs, then they appear as green on OS landranger maps. If they've got black and white signs, then they appear as red on the map. That's exactly the same as OpenStreetMap has it.

And besides, when you survey a road, you have to actually go and visit it - for the GPS track or for the road name or number. What can be simpler than looking whether the sign has a green or white background whilst you're collecting your GPS track??