Oh, good grief. Here we go again.
Contrary to popular perception and a lot of careless news reporting, embassies are the sovereign territory of the country in which they are located, NOT of the country whose diplomatic mission is housed there. That is why an office building can host an embassy on some floors and, say, a bank on others.
The reason for the misperception is probably that the Vienna Convention states that the local government foreswears the right to enter an embassy, and diplomatic immunity protects the diplomats working inside. However, this does not mean that that space is somehow transmuted into UK (or other) soil or legal territory for purposes of law enforcement.
Imagine, if a person should enter the British embassy and kill someone, who do you think will arrest him? Correct answer: the Chinese police. They cannot enter the embassy without the Ambassador's permission, but with that permission they will go in, take away the miscreant, and then try him in Chinese court under Chinese law. Or else the embassies security officers will deliver him to them outside the embassy gate. Imagine, if you can, the utter nonsense of the fellow being hustled onto an airplane and flown to London for trial.
I am a US diplomat and consular officer.
Here is the Vienna Convention on Diplomatic Relations. Notice the absence of claim that an embassy is the sovereign territory of the country that occupies it, only that the embassy is protected, see article 22 and on.