WRONG.
UK law applies in all foreign embassies in the nation.
It's only amabassadors who are immune to prosecution (and only under certain caveats).
Otherwise, you could literally sneak people into a foreign embassy, torture them in front of an open window, and nobody would be able to stop you.
The "foreign soil" thing is NOT TRUE. Stop perpetuating it. The premises are subject to UK law at all times.