Man comes of age, travels to London, meets woman ...

Please help me ID this film. I saw it in my childhood, going on 50 years ago.

It was about a young man coming of age. On his birthday he travels into London and meets an older woman, they sleep together but will never meet again. I remember the old British Rail slam-door trains in the terminus and the final scene with a balloon floating up into the station canopy.

I'm guessing by the train age it was a 1970's movie. I remember it as it's probably the first recollection I have at feeling emotion watching a film!

Thank you for helping.