UK dopers: origin of "No sex please, we're _____ " phrase

I’ve heard this in various Britcoms forever. Its even in the original HHGTTG radio plays. So what is it from and what does/did it mean?

There was the 1973 film. I don’t know whether there are earlier references.

From here:

One gets the sense that the title might hearken back to some other phrase, perhaps common among traveling Britons, No xxxx please, we’re British. If that’s what the OP is getting at, rather than the play I have cited, I have nothing. But I’d like to know, too.