I had an interview recently where I thought things were going well, until asked about availability, and I had to say that I wasn't available immediately because I've booked travel home. Then the interview was over, because they needed someone to start ASAP.

Is there any protocol to say at the beginning of an interview something like "By the way, just to let you know that if you were looking for an immediate start, I wouldn't necessarily be available, but I'll be available in January if that makes a difference?" or something similar. Just to avoid getting to the end of a good interview and then realising your availability and the employer's doesn't match. Or would it be seen as rude and potentially starting the interview off on a bad note?

This is in England.