What am I expected to say about teaching plans in an interview (UK)?
You can always ask if they can give you any further guidance about what they are looking for in the 'teaching' part of the presentation. I imagine this is also somewhat field-dependent: what is expected in a humanities department may be rather different from what is expected in a lab-based science area. However, generally people:


Summarise what teaching they've done in the past, highlighting anything impressive (positive student outcomes, innovative approaches to teaching...);
Identify which aspects of the department's current courses they could contribute to, and particularly any areas where the department is currently lacking expertise (tactfully!);
Perhaps sketch an idea for a new course that they'd like to put together if they got the job.


In a 15-minute talk, one slide on each of the above is probably a reasonable starting point.

In response to OP's clarification that the advertised position is specific about the module to be taught: I think the same general points apply, but you should focus a bit more specifically on that module: Why is it important? How would you approach teaching it? What makes your version of that module better than the other candidates' versions? However, I don't think it ever hurts to show versatility: don't focus on that module to the exclusion of all others.