It is because congress and law makers don't make the US federal agencies accountable for their actions. A law does not exist if there is no punishment for violation of said law.

It's not actually illegal for the CIA to spy on foreign embassies.

The US was caught bugging the UN, and we tapped Angela Merkel's phone. The intelligence agencies are, more or less, doing exactly what they were set up to do.

And the other guys are bugging us in return (see: mysterious cell-site simulators popping up in DC, etc).

The CIA runs a drone warfare program, bugging one embassy is not going to turn their stomachs.

Which international laws would you be referring to in this case?

The vienna diplomatic convention according to which the UK should protect the embassy from intrusion

https://en.wikipedia.org/wiki/Vienna_Convention_on_Diplomati...

The Vienna Convention isn't super clear on what counts as "intrusion", but I suspect it's generally read as physical intrusion - you don't send people in to raid the place. Spying on embassies has a long, storied history.

What exactly do you think intelligence services do?

They do this, that's their job.

Well they did a dismal job of it (i.e. were suspected and got caught). If justice prevails none of that evidence will be admissible, and the extradition efforts have likely been put at risk.

Listening in to privileged conversations between a lawyer and their client would be viewed quite poorly by any judge.

There is no institution to enforce them - the best we have is the UN Security Council and the International Court of Justice, the latter of which will only arbitrate disputes between consenting States.

No State is going to hold each other accountable when they're all doing the same thing, and nobody would ever sign up to some sort of supranational authority.