Check with Loganair at Benbecula re fuel. Not sure they still operate Islanders there, so might not be any need for AVGAS. Stornway is much bigger than Benbecula, but any visit to the Western Isles is well worth it.
I've done Stornoway - Blackpool - great scenery and you stay clear of the Scottish TMA. Oban is a short way off track for fuel on the way and may be preferable if you want to keep clear of too much water.
Watch for the mountains and have a great trip.