Leave in a box
Assange could leave in a box, literally.
If they put him in a large box which is marked as a diplomatic pouch they could take him out of the embassy to a ship or plane and take him to Ecuador. Once outside of UK airspace/waters he could safely leave the box.
UK authorities couldn't legally open the box under the Vienna convention.
Less of insane option would be to designate Assange as a diplomatic courier to take a diplomatic pouch to Ecuador. Although the UK has to officially accept designated diplomats for them to be allowed to operate in the UK, an option Ecuador has already tried and failed, under the Vienna convention diplomatic couriers can be designated ad hoc so the UK can't refuse him if he is issued with the correct papers.