No, the UK is sovereign inside the Ecuadorian embassy. The authorities are prevented from entering by international treaties that they've signed.
If international treaty obligations block sovereignty, then the UK isn't sovereign within the UK either (!), since there are certainly international treaties signed by the UK which prevent the UK authorities doing certain things within the UK. For example, the UK authorities cannot usually arrest diplomats, regardless of whether or not the diplomat is in an embassy.
> If international treaty obligations block sovereignty, then the UK isn't sovereign within the UK either (!)
Yup. International treaties are an exchange of sovereignty for something else. The general public imagine that XIX-century-style nation states still exist, but they haven't for a long time. I suspect the USA's rhetoric of patriotism, especially post-9/11 exacerbates this view. Even the USA shares a lot of its sovereignty with external entities (gasp!).
The only XIX-century-style nation state left might be North Korea, FWIW.
Sovereignty isn't binary, and states and laws are one of many social constructs. Discussing the complexities is interesting. Purely semantic arguments is not.
I was responding to OP's claim that Assange was not "in the UK" when he was in the embassy. That's false on any reasonable understanding of those words. It would only be true if embassies were in fact foreign soil.
If international treaty obligations block sovereignty, then the UK isn't sovereign within the UK either (!), since there are certainly international treaties signed by the UK which prevent the UK authorities doing certain things within the UK. For example, the UK authorities cannot usually arrest diplomats, regardless of whether or not the diplomat is in an embassy.