The medium used in air filters has a progressive pore size through the thickness, wide on the dirty side and smaller on the clean side. This is to maximise the dirt capacity while minimising the restriction. Large particles get caught on the dirty side, smaller particles go further through before getting trapped.
Air boxes are often designed to direct incoming air at one general area of the filter element so that heavier dirt gets deposited in one spot, leaving much of the element pretty much clean. This is intentional, pile up the dirt in one place and the overall life is extended.
Generally speaking you will make very little difference by trying to clean a paper/synthetic type air filter. At best you get some of the big loose stuff off the dirty face, but this doesn't usually contribute much to the overall restriction. Smaller stuff caught within the medium doesn't usually come out by back-flowing.
I wouldn't go as far as the suggestion that an air filter is not really needed in the UK, I think that was probably tongue in cheek knowing Culzean, but certainly UK air quality is pretty clean. However any dirt contaminating any sort of hot mass air flow sensor (MAF) is not good, so should be avoided.
The only thing I draw a line at is the oiled re-usable type of air filter. Fine for competition use to stop birds and small children going into the intake, but not for a road car.
Based on experience of developing air filter systems for cars, I would suggest that 25k miles is a perfectly sensible life for a typical UK car air filter. Even an OE Honda air filter thus works out at around the £1 per 1k miles, compared to around around £100 per 1k miles for fuel. Treat air filters as a consumable and a cheap one at that.
I quite like the Mahle air filter technology if you want an aftermarket make, Opie do them either direct or via their ebay shop. I don't have connection with Opie but have had connection with Mahle professionally, though I try to remain impartial.