I'm amazed at the lengths UK runways have apparently reached in rumour and legend!
As Speedbird 2946 says, the longest runway now or ever in UK is London Heathrow's 27R. Bedford was the longest 'military' (OK, RAe) airfield. Speedbird's figures are correct. There has never been a runway of 14000ft in UK! The large number of runways extended in the 1950s as part of the USAF Korean War/Cold War military build up were nearly all taken out to figures between 9000ft and 10000ft. Finningley was just under 9000, Elvington was 9800. It was, I believe, restored to its full length when the RAF resurfaced the airfield in 1989/90. Sculthorpe was unusual in that its extension to 9000ft (not 14000) came between 1944 and 48.
Manston, along with Woodbridge in Suffolk and Carnaby in East Yorkshire, was reconstructed in 1943 to have a single runway of 9000ft by 750ft width (not 300) expressly for emergency landings by returning heavy bombers. If you look closely now, you'll see that the northern parallel taxiway sits on the same hard surface as the currently marked 200ft wide runway. The old runway forms the foundation for both.
St Mawgan (the eastern extension and replacement for Trebelzue airfield) is 9000ft by 300ft; I believe this is the widest runway in current use in the UK.
RAF Macrihanish (now known as Campletown) is 10000ft by 150ft, as reconstructed in 1960-62.
I believe the longest runway never used in the UK was at RAF Chelveston, in Northamptonshire (now the site of a radio relay station). Chelveston was extended to somewhere around 3600m (10800ft) in the 1950s, but no aircraft ever used it (although there are rumours of one crash landing). The runway was broken up in 1977.