Re: Leave in a box
Incorrect. One, under the Vienna Convention, a diplomatic pouch can ONLY be used for official diplomatic communications. Anything that doesn't conform to that specification can be considered suspect and NOT allowed the immunity. IOW, the UK can say they're "taking the mick", search the box and be proved right if there's anything in it other than official diplomatic documentation. And it's highly unlikely the Ecuadoran embassy would be keeping enough official diplomatic documentation at any one time to require a box big enough to house a human; SOP would be to ship the documents more often: in smaller, more manageable sizes.
Two, the UK doesn't adopt the entire Vienna Convention verbatim. What they obey is the Diplomatic Privileges Act, 1964, and that act specifically grants the UK the power to override and revoke a diplomatic privilege (it's in section 3).
PS. This ALSO means the UK can deny Assange status as a diplomatic courier. Under the DPA, the UK holds final say over matters within its borders.