i always believed that an aircraft regardless of where it is, is an extentsion of the mother country.So even if the aircraft was sitting at the gate in SA its still considered British soil.
2 Cases in mind, 1) The US govt stopped a SwissAir plane from taking off at JFK yrs ago since they had creditable info that Marc Rich (Yes the same guy that Clinton pardoned) had all his papers that the Govt had court order to be turned in on that flight. The Swiss plane returned to the gate but denied any US Govt offical from boarding and removing the boxes of files as it was Swiss Territory and the US complied. Rich paid over a mil a day in fines until the standoff ended and the govt got its way. The point was the plane was recognized as Swiss property although it was at JFK.

2) many countries make it known to their own CIA etc employees, if you get in trouble and being chased just get to an airport and get onto any US carriers plane, its the same thing as being able to gain entry into an Embassy or Consulate. probably alot harder today than B4 9-11. Israel had its Mossad agents use this process at times in the past. Agian its not full proff that the plane will be allowed to take-off but it gives that country some breathing space to try and work things out.

so in regards to the 2 items above being on a BA craft UK laws should take the day and not SAs. Should they have been allowed to kiss, if it was a peck and nothing sexual i guess, no different than a hetro couple. But under no circumstances should they have gotten into a debate or worse with the Crew.They should have gotten witnesses to what occured and sued. I have run into some FAs that were anti my religion, not much I could due, I did complain when I landed, since at no time time did she/he say the reasoning behind their statements, it was obvious but Not outward. Had they voiced anything in particular that would have been different. OK the FA is against gay people Nothing will change that, but thats no license to go off the deep end either. i dont feel its right that the FAs can get away with it during the flight but such are the laws. They can simply say they feel you are a threat to the flight and convince the captain of that and thereby the Captain can make an unsheduled stop and YES get you removed from the aircraft and force you to pay all charges including the dumping of the fuel and refueling. yea you can sue but it Aint worth the hassle.
So one can be right yet its best to shut up and wait till the plane reaches its destination to deal with the problems.
Im talking from experience of being off boarded before we took off in another country. And being read my 'keep my mouth shut' on another flight months later that had an FA in 1st with me that was working the first flight. In both cases the Captain just stood by what ever the FAs told 'em, I got a kick out of it. I hope this guy prevails, hopefully there wont be a next time but if there is, hopefully he learnt his lesson to as hard as it is, to keep his frustrations to himself till the plane has landed or to at least not raise his voice or threathen any of the CREW in any way. Should it have to come to this NO!!!!!!!!!!!!, but the rules in the Air are different than on the ground, should they be NOT Important THEY ARE.