> **what international law(s) are at play here?**

In the case of Assange being removed from the Ecuadorian embassy with the consent of the ambassador -- only the Vienna Convention.

Subsequent likely proceedings are not covered in this answer but, for example, the Universal Decaration on Human Rights can apply to extradition proceedings.

> **Can the mere act of withdrawing asylum really be illegal?**

International law applies where someone outside their home country applies for asylum in the country where they are present.

Some international laws do cover the situation where asylum or refugee-status is withdrawn or where someone is expelled from a country.

1928 Convention on Asylum
-------------------------

The [1928 Convention on Asylum](https://www.refworld.org/docid/3ae6b37923.html) was signed by Ecuador (but obviously not by the UK for geographical reasons). It says

> Signed in Havana, February 20, 1928, at the Sixth International Conference of
American States

...

> Article 1.-
It is not permissible for States to grant asylum in **legations**, warships, military
camps or military aircraft, to persons accused or condemned for common crimes,
or to deserters from the army or navy.

> Persons accused of or condemned for **common crimes** taking refuge in any of the
places mentioned in the preceding paragraph, shall be surrendered upon request
of the local government.

Which suggests someone who is sheltering in a legation should be surrendered if they were accused of the common crime of failure to appear in court after being granted bail.

It doesn't explicitly say anything about withdrawing asylum though.


---

1951 Convention Relating to the Status of Refugees
--------------------------------------------------

Someone who has committed a serious crime is excluded:

> F. The provisions of this Convention shall not apply to any person with
respect to whom there are serious reasons for considering that:

...

> (b) he has committed a serious non-political crime outside the country of
refuge prior to his admission to that country as a refugee;


A crime punishable by a year in prison may be serious enough.

Of course there is another reason this does not apply to Assange --  he was not in the country of refuge. Being moved from one part of London to another part of London does not constitute expulsion from a country. Certainly not from Ecuador.