Of course MAN was the last airport to have a new "full-length" runway, can't remember when it opened though.
On my last few departures from Ringway noticed departing aircraft being in a queue waiting to take off and being held up by a landing aircraft (as if there was only one runway) while the second runway was unused!
No idea about the runway built in 1946, those making the point never mention where it is, nor do they define "full-length". With so many RAF airfields becoming available for civil use
at that time, can't imagine why any
new ones would need to be built. Now, however, we need two more, and that's just at LHR!
AFAIK the two longest runways in the UK are at LHR and, again
AFAIK, were extended in the 1970s.