Curly, where would you have come back to if your Dad's house had sold last year? Just a thought.
Yet again, what was the contract you or your Dad, made with your friends? Was it written or with an Agent or solicitor?
If they were aware that there was a possibility that you may be returning, was 2 months noticed agreed with them?
Lets call a spade a spade, they probably thought they had the house for as long as they wanted it and are now a bit pee'd off about having to move.
Was there anyone in the UK collecting rent from them or overseeing the house in general while you were away? As asked above I guess, an Agent of some sort?
If there is, then the Agent would have to advise them - in person would be a better idea, and give them it in writting - of your intent to return.