Hello!
Not a stupid question at all! The short answer is that it is absolutely fine to talk about a deal from another office - the interviewers will mainly be testing:
- Whether you understand what the firm actually does
- Whether you can explain why the deal interested you, and
- Whether you can link it back to the practice area or skills relevant to the London office
If you wanted to be clear in your answer, you could briefly acknowledge that the deal is from another office and explain how the same practice area or client work is relevant to London (e.g. cross-border nature, collaboration between offices, similar clients / sectors).
What matters more is that you can talk about the deal commercially and with genuine interest - a well-understood and well-explained deal from another office would land better than a London deal that you only half understand.
Best of luck with the AC!!