I spent 13 years as a Policeman at heathrow, and up till about 1996 if we were called to a foriegn registered aircraft we couldn't do very much about anything that happened on board. If an offence continued when the parties got off we could possibly do something then, for instance could arrest a drunk when he crossed onto the airbridge, provided he was either disorderly or became incapable, possibly where the crew dropped him.
In around 1996, can't exactly remmeber the year but that would be close, the goverment passed the Civil Aviation(ammendment) Act.
This gave UK police juristiction in a forgein registered civil aircraft, provided there was an equivenlent offence in the coutry of registration (Which the accused had to show there wasn't) and The airport the aircraft landed at was the first point of landing since the offence took place.
I understand that this act was passed in responce to an international convention, so I belive that many other countries have a similar law.
Of course prior to this act we could do nothing and the aircraft crews seemed loath to take any action in thier own country. The only exception I recall was where the crew of an airline just to East of Iraq detained a passenger who said something about that country's political system as he got to the door on the way out. He wasn't alowed off and we were not allowed on.