O2 vs Vodafone coverage in rural areas

Frank, good question, however in my experience both of them have their dead spots and not much to choose between them. The absolute best is a Manx Telecom SIM product called Chameleon (or used to be) that roams between all the networks (O2, Vodafone, EE etc) and gives you the strongest signal. I had one for a while in a mountain dedicated phone as the former Chairman is a friend. I fell out of love with it when we all discovered it doesn’t roam, so was caught out when ski touring in France / Switzerland.