And the UK considering entering an embassy is a definite no-no under the Vienna Convention and will set a terrible precedent.
Revoke their status as an embassy, enter it and arrest whomever you want, and take whatever documents you want, then leave and declare it to be an embassy again. Since it wasn't an embassy at the time, the Vienna Convention doesn't apply. Neat and tidy. And a complete farce, ethically/morally speaking.