I'd guess its a toss up between the VC10 and Dominie,
I think the Dominie entered service in 1965, the year before the VC10, so leaving aside BBMF and Boscombe, which may have a Sopwith Camel on its books for all I know, I guess it is the oldest in regular RAF service. Some chat about it on the 55 Squadron visitor to NCL thread below. Funny, I still think of it as a shiny new aeroplane, which it certainly was at Strad after Gaydon's Varsities, but time marches on.