In reply to Pero:
It depends where you go in the Highlands.
Not a helpful answer, perhaps, but there you are. I know places where the only network to give coverage is O2. I know places where the only coverage is Vodafone. Last I heard, Vodafone have the contract to ensure complete coverage for the railway lines, so Vodafone works near railway lines.
All other things being equal, phone networks based on 900MHz will give signal further from the base stations than will phones based on 1800MHz or 2100MHz, so O2 and Vodafone will work further away from the nearest base station than will EE, Orange, T-Mobil.
In short, if you want to go to a specific place or places, then find out from the locals what networks work there. Otherwise I would go for O2 or Vodafone.