It's scarce.
I know petrol is available at Portland Marina - don't know about Weymouth.
nothing shoreside at Westbay or Lyme -maybe a petrol station some way from the harbours.
Nothing at Exmouth harbourside
Diesel only at Torquay currently - Brixham the same.
Petrol at fuel barge is Dartmouth, then Salcombe,at a premium
Can you carry some 20l cans and refuel from those?
Alternative is to leave the coast and go straight Portland - Dartmouth (will take you outside he normal 12 mile coastal insurance cover).