Promini has an informative article on the wonderful world of cabin air filters. It may not be as exciting of a topic as that latest JCW spoiler but it’s probably more important. Here’s an excerpt:
MINIs are equipped with a small A/C cabin filter element. The filters are designed to trap all dust and/or solid particles. This is a major improvement over the window screen style filtration of the 70’s and 80’s. Unfortunately most car owners are not aware that this item even exists and requires annual replacement, and more often in dusty areas that have high vehicle population. The only indication that you would experience is that the A/C does not seem to have the force that it did when the vehicle was new. Similar to a dirty engine filter, restriction reduces airflow and volume.
You can also check out installation instructions here.