Why is that a crime anyway. It doesn't harm anyone
So, sex in public isn't necessarily illegal in the UK as long as you are discreet and not causing a nuisance. Police are generally sensitive to areas that are known PSEs (Public Sex Environments) and will only intervene if there is a complaint.
However, engaging in sexual activities where you are likely to be seen by the general public will potentially open you to various decency and public order charges if they determine that your activity has caused harassment, alarm or distress.
I cruise quite a lot and the police are aware of the more popular cruising/dogging spots and will generally leave them alone. They even occasionally have outreach programmes with charities like the Terrance Higgin's Trust where they will set up liaison points and STD testing as well as giving out free condoms and tea and biscuits.
Fucking in a tube train would be a public order violation though, hence the charge.