If you're the British government, and your citizens who are legitimately in a foreign country, travelling on valid passports and doing nothing wrong, are being forcibly detained, and if the local authorities are not stepping in to try to resolve the situation in some reasonable way, how do you treat that situation?
On the one hand, you presumably want to be diplomatic when dealing with another nation. Certainly you don't want to escalate any potentially violent stand-off unnecessarily.
On the other hand, the basic facts are that those are your people and they're in trouble through no fault of their own. Isn't your first responsibility to protect them and get them safely home if need be? At some point, there will inevitably be a very serious discussion about how far you need to go to do that, and that discussion will consider deploying scary people with weapons in another country, which most certainly is an act of war.
