Just a reminder, but the uk does not equal England; in case you forget, there's wales, scotland & n. ireland and there's plenty of out of the way places in those three countries for such things (One of the worlds biggest acid labs - the one of operation Julie fame - run by Kemp was situated in Wales. They're a lot harder to hide than a meth lab)
Because the other components needed to synth meth are watched much more closely (eg methylamine) because it's also used for making MDMA or they stand out like a sore thumb (eg red phosphorus) on a chemical inventory. There are ways to do it using a synthesis that doesn't use red phosphorus or methylamine, but it doesn't seem to be very well known, hence most of what's produced is amphetamine (uses ammonia rather than methylamine, which is piss easy to get)