Well, I guess nations do that all the time. Certainly I've heard of it in the last year or so.

  https://www.independent.co.uk/news/world/middle-east/turkey-erdogan-president-us-ambassador-visa-latest-a7993116.html
  http://www.telegraph.co.uk/news/worldnews/the-pope/11527987/Pope-Francis-refuses-gay-French-ambassador.html
  https://www.radionz.co.nz/news/national/254049/israel-won%27t-recognise-nz-ambassador

In a legal manner (and not because I'm a UK citizen) I presume the host nation can decline and then the proposing nation can find another one or send them anyway but risk that he falls foul of the law and the host country does not recognise any diplomatic immunity. In this case though, if the UK declines then he is still stuck in there with no way out.

I think its kind of strange really that we have spent all this money pursuing this guy for what was a minor warrant (that they wanted to speak to him) which has since been cancelled and think that we (the UK) should just say enough already and turn a blind eye to him leaving on a jet plane. Oh yeah, but the strange part is that it really seems as if its important to somebody that that doesn't happen.

Normally a country can reject a diplomat's entry into the country. Which makes sense, given the extent of diplomatic immunity. Therefore it does make sense that one cannot willy nilly change a person's diplomatic status while in country.

Edit: delete repeated words

The other nation doesn’t get to decide what happens in the UK because the UK is also a sovereign nation.

Although it would be unusual, impolite and probably unwise, if the UK decides to enter the embassy by force and just take Assange, there isn’t a whole lot Ecuador could do about it, apart from making a big fuss. Ultimately it is the UK who decides to allow Ecuador the right to have an embassy on their soil in the first place and they can come back on that decision whenever they want.

The host country has the right to accept or deny a diplomat's credentials. Here's the US Ambassador presenting his credentials to Queen Elizabeth.

https://www.youtube.com/watch?v=S4E9LMK46oo

think that's the norm, really. Usually it is more a rubber stamping formality (Russia / Trump's US-to-Russia diplomat pick), but I imagine a country could reject a diplomat, especially if said the host country has a previous issue with the diplomat in question.

I'm a bit shocked at how brazen it is, honestly. Perhaps, if anything, it implies that Britain doesn't view Assange as a big issue anymore as doing what Ecuador did last month would almost certainly be viewed as a diplomatic slap in the face otherwise. It almost reads like their response was "haha--no"

Might makes right.

They did until 2015 [1]. The UK government estimated that cost over GBP 12 million.

[1] http://www.bbc.com/news/uk-34508500

Search for the same phrase? There are plenty.

Also, I don't need a smoking gun because I'm not bringing anybody into a court for trial.