Andy
I don't think there's any problem with your met skills, you've just experienced the very local nature of Exmoor weather. I spend a lot of time in the area as my mother lives near Williton. You can leave the Minehead area in glorious 28C sunshine, hit the Somerset/Devon border and all of a sudden be shrouded in fog. Lynton & Lynmouth will be overcast and foggy but get a few miles further down the coast and the sunshine will return.
I suppose it's not surprising considering the moorland's close proximity to the Bristol Channel, which has it's own weather perculiarities. When I used to sail in the channel, even if the weather forecast stated force 1-2, 30C and sunny, I'd still pack three sweaters and my oilskins.