(Public Switched Data Network)
In this country, PSDN exclusively refers to BT's PSS (originally Packet Switch Stream (and any Mercury competitor?), which is (are) an X.25-based network(s). From BT you can buy (at exorbitant cost) leased-line connections to PSS at various speeds - contact their sales people. You can also dial-up PSS at slow speeds - PSS DialPlus.