As said in earlier posts, air filters get a fairly easy life in the UK, the air quality is pretty decent these days in all but the most congested traffic areas.
I've done air filter development in the long distant past. The filter life is based on an increase in pressure drop at given airflow rate, and in reality in the UK 25k miles is usually perfectly OK. The life DP increase is often taken as around 5"H2O in old money, or around 1.5% of atmospheric press. You can take that as roughly the decrease in max power if you wish, and you'd never notice 1.5%.
Air filters used to be paper fibre (cellulose) based, it was laid down in progressively finer layers so the dirty side was open fibres to catch big dust, then it got finer through to the clean side to catch smaller stuff. The clean side of the paper was usually marked with a stripe. The grading maximises the dust capacity for a given DP.
Modern filters are made from synthetic fibre but made with the same principle of graded pore size, the big difference being the much greater thickness for the material so more dust capacity generally and better life performance.
I'd be fairly relaxed about using any good branded filter, Bosch/Mahle/Mann etc from a known supplier, I'd avoid sourcing from anonymous fleabay sellers, there are loads of fake parts around and considering the relatively low cost and ease of sourcing from good known retailers then why bother.
The one thing I personally wouldn't use is one of the well known re-usable oiled type "performance" filters, basically they don't generally catch much fine dust once the oil disappears, and the oil can mess up MAF sensors.
