I'm unsure, do we have an official Ambassador, ir is that simply what is he known as - or was, now that he has gone.
The two polarised trains of thought - are there any kind where Brexit is concerned are as follows -
Sir Ivan's opponents think he he an Ee-Ore character, always saying you can't do this, or this rule stops you doing that, and they hold him responsible for talking Cameron out of asking for a decent deal when he last went over there to talk to the EU.
His fans say he was well-respected, knew a lot of valuable contacts, and understood the massively complex rules concerning trade deals from which we have to extricate ourselves.
Everyone seems agreed that Mrs May wants her person there, not Cameron's whom she inherited.
As to the reason why he has gone - it seems that he was perfectly happy behind the scenes, but got thrust into the spotlight when he pointed out that it may take ten years to get a deal sorted. The problem there was, he was pilloried for that view when it wasn't actually his - he was simply passing on what the Eurocrats told him, but he got saddled with being a doom-monger. Sources reckon it was this that made him decide to walk away.
As with Brexit itself, everyone will be spouting about the effect, long before the effect is actually known.