Not read all the replies, and the main caveat is that most brands and models have horses they work for, and fitters that make them work well. In quality terms they're not like Ideal, completely different, mainly produced abroad and assembled here and certainly used to have batches with quality issues, though I've not heard of any for a while. Ideal are a bench made saddle and should be (not been to the workshop!) assembled by one person, K&M are generally assembled on more of a production line. Doesn't mean they're not decent value, but be aware of what you're buying.
They are partly popular because they are the main brand for training SMS fitters, to be an SMS fitter you have to stock a wide range of English brand (not necessarily genuinely English made) saddles. The SMS has been a body for saddle MAKERS for a long time, they added fitters on, and it is used to support English saddle making. If you've been trained on them, you'll want to fit them.
I fit very differently and am not a fan (but then I only fit two brands, brands I love!).