How would it harm UK if another embassy accepted Julian Assange if he was bailed, since they would not be paying for his accommodation ?

He's currently serving a sentence for jumping bail.  Sweden has dropped all charges against him and the extradition treaty with the USA is dead until they hand over Anne Sacoolas.  Once he's served his sentence, he's free to do what he likes.

I doubt they'll give him bail, their objective is to disappear him 

They spent a fortune waiting outside the Ecuadorian Embassy in order to nab him when he was evading arrest. I doubt any other embassy would be willing to grant him asylum given his behavior, but if there was no reason to re-arrest him he can go where he likes, unless he is deported. Currently he is detained because he is a proven flight risk pending the USA's appeal against his non-extradition.