They don't, quite often. A GP has so much to keep up with, having to know something about just about everything... And as you're finding yourself, there's lots of contradictory information, and most of it goes against what the government officially advises, usually. If they could take a week or two and do a deep-dive into the material, maybe.... But what NHS doc has any time to do anything without it cutting into the time they can care for patients?
My doc told me she didn't know what I was on about, but I was welcome to give it a go and be a guinea pig, as long as I got blood work done to keep on top of things. I'm the only one in the practice who went low carb, and far as I know, the only one who got back to normal numbers in these past 7 years. She was supportive, and I know I got VERY lucky to have her.