If National Highways are being too vague about the actual speed limit of a roundabout (it isn't as if you'll reach the limit on one anyway) then I'd suggest that we should tag roundabouts as 60 mph where one or more of the arms are single carriageways (e.g. the A4102 here) and 70 mph where all of the arms are dual carriageway (e.g. M25 J2/Darenth Interchange). It might be better to do a blanket 60 mph rule per what Nathan_A_RF says to make it easier since the latter example is much rarer than the former example in the UK at least. Legal documentation or design guidelines would be very useful in this case as a way to determine how we should map it on OSM, which I think Nathan has alluded to here.