I would imagine the letter from his landlord would need to be send via post, as anyone could forge an e-mail saying it was okay for you to live there. Remember also, you would need to show that the property your friend rents is a suitable accomodation for the additional person(s) without making it overcrowded.
If you're living in London, I wouldn't think your owning a property in Scotland would have anything to do with anything unless you were intending on living IN Scotland AT that property.