Come on folks, think about it. These days the flight deck on an airliner is closed off. They get a report of something going on down the back the first thing they will do is report it - they have no idea how serious it is. From that first radio call then QRA will be launched and it won’t come back until they have seen that jet on the ground. What would happen if the crew called it in and then, under duress, said that everything was now fine and that it actually wasn’t.
Looking at the news report on this the woman wasn’t exactly ‘just a bit shouty after a couple too many wines’ - she allegedly tried to rush the cockpit, made several death threats and required 6 people to restrain her. How do you know that she isn’t one of several on board - you don’t. The airline crew made the right call and it was correct to send the Typhoons up to get “eyes on” to make sure things are OK. Finally, if there was an airliner hijack and it looked like there was an intent to pancake it into London then a decision to bring it down might have to be made - it’s a scenario that is practiced many times. This 2015 news clip shows that final resort: