Obvious ploy but...
The UK has no right* to deny Assange or any other person from being a diplomat of another country. It is up to the other country to decide who are their diplomats. The UK has the right to reject the chosen Ambassador to the UK, but not the diplomatic staff.
A diplomat is entitled to their privileges as soon as they correct ministry is notified.
1 .Every person entitled to privileges and immunities shall enjoy them from the moment he enters the territory of the receiving State on proceeding to take up his post or, if already in its territory, from the moment when his appointment is notified to the Ministry for Foreign Affairs or such other ministry as may be agreed.
However the UK can deny him a visa so he would presumably be ejected from the country if he left the embassy.
*According to The Vienna Convention on Diplomatic Relations.