If Ecuador say he's a diplomat then he's a diplomat. The UK Govt have no say in the matter. They can declare him persona non grata but they can't take away his diplomatic status or immunity.
As Craig Murray posted on his blog:
"The Vienna Convention on Diplomatic Relations – to which the UK and Ecuador are both party – is the governing international law and determines the obligations to respect diplomatic immunity. It is crystal clear (Article 4,1) that the need to obtain agreement in advance of the receiving state only applies to the Head of Mission – ie the Ecuadorian Ambassador. For other staff of the mission the sending state (in this case, Ecuador) “may freely appoint” the other members of the mission, (Article 7), subject to provisos in Articles 5,8,9 and 11. Plainly the only one of these which applies in the Assange case is Article 9. Julian Assange is persona non grata – unwelcome -to the UK government. That is a legitimate reply to notification, but comes following the appointment; it does not pre-empt the appointment."