At the start of the year, I applied to five PhD in AI programmes at top UK universities. With the help of a supervisor, I received an offer from one of the universities and was rejected by three universities. Having not heard anything from the final university, I accepted my offer. In June, I was rejected by the fourth university, which I suspect hasn't even looked at my application—I believe they had reached their shortlisting quota before getting to my application.
I am supposed to start my PhD in September, but I have just received an interview invitation from the fourth university because someone has dropped out of the programme.
Is it acceptable to reject my PhD offer after accepting it for another PhD offer? Both universities have strong reputations, and both supervisors have been pleasant. The main advantages of the fourth university are that it's in London, where many large tech companies operate (potential for collaboration), and it's close to home, which means less travelling—I hate travelling. If it is not acceptable to reject my PhD offer, how should I turn down the interview with the other supervisor?