I think Daniel Roseman has it almost right, and what you saw was actually The Quatermass Xperiment, the 1955 theatrical film based on the 1953 television miniseies. The story follows the same outline as the six-episode serial, described in this was by Wikipedia:
Three astronauts are launched into space aboard a rocket designed by Professor Quatermass, but the spacecraft returns to earth with only one occupant, Victor Carroon.... Something has infected him during the spaceflight, and he begins mutating into an alien organism which, if it spawns, will engulf the Earth and destroy humanity. When the Carroon-creature escapes from custody, Quatermass and Scotland Yard's Inspector Lomax..., have just hours to track it down and prevent a catastrophe.
The film, like the television serial, was a success, and it launched Hammer Films as a major player in the British horror film business.
There are two major reasons that any version of Quatermass shown on television in the 1970s or 1980s would have to be the film version. Firstly, the Hammer film version is feature length and thus easy to slot into a television special slot. The television version is longer, approximately three hours. Moreover, unlike the television version, the Hammer film still exists in complete form. As is the case for many BBC television productions from before 1980, most of The Quatermass Experiment is lost. (This phenomenon is famous in connection with Doctor Who, but it actually affects hundreds of BBC-produced programs.)