Agree with you on Hunter wellies, we definitely use them a lot, partly because they look good with a whole range of outfits, and partly because the girls find them really comfortable to wear, both dry, and full of water or mess, so any welly shoot now tend to favour them unless we specially want a more "industrial welly" look. They're also easy to put on and take off, some wellies can be a nightmare to get off once they're full of water.
Other brands common in WAM? Adidas comes to mind, they're one of the No1 sports brands in the world, we've done plenty of scenes with our girls in Adidas tracksuits (several to be released), and I've seen Adidas mentioned quite a bit by others too. Plus the shiny nylon shorts that are almost a fetish of their own are an Adidas design.
Various forms of Nike trainers seem to be popular, as are Vans shoes.