But yes, if there is a central reservation (defined as a section dividing the carriageways constructed of a different material than the road - grass, gravel, mud, but NOT JUST tarmac with paint), the speed limit for the road is 70mph for cars if the NSL applies.
I don't get how people can do a driving test and not know this stuff tbh, around here the is a dual carriageway with a grass verge in between the carriageways but no barrier (it was the first dual carriageway in the country so quite old) and the amount of people you see going down it at 60 or sometimes lower because their thick is silly (may also be because its horribly surfaced and in some corners the camber goes the wrong way but /meh )