Like Don D Cake I know the area well as I live & fly there. There is lots of localised weather due to the proximity of plenty of bits of high ground, Exmoor being just one, and the sea all the way round the edge. A southern route could be even worse. Dartmoor is higher and nastier than Exmoor and the coastal strip can be a bit dodgy too, and far too close to high ground for comfort. I'm afraid you just pick the best day possible and hope for the best. BTW when the sun does shine here it's spectacular.