What is the timeline for UK postdoc applications in math?
The timeline varies from one place to another. Some Universities will compose a shortlist in a week after the deadline and call candidates in for an interview a couple of weeks later. Following the interview, the final decision can be made in a couple of days.

However, it is not uncommon for the shortlisting to take several months. For various reasons, the final decision after the interview can also take longer. I received my postdoc offer ~3 months after the interview date.