I hate to say this, but the mortgage company can change the T&C whenever they want. Several years ago we were in a similar position, and had a renter ready to move in, agreement from the mortgage company in writing etc. Then they changed their policy and sent us a letter saying that we couldn't rent out the house.
You will almost definitely need a buy to let mortgage, and they can cost a significant amount more, you'd need to apply for one and get approval, then pay up your current mortgage complete with any fees to early payment.
There are mortgage brokers who specialize in mortgages for people living overseas, so that would be worth looking into. A good broker should be able to find a mortgage that works without costing you a fortune.
Also - banks are getting more wary of customers who live overseas. We live in the US and it looks like we'll have to shut down our UK accounts and move to off shore ones, which cost money.
Basically - every time you move country, and move money/mortgage etc it costs you money. I would budget for 5 - 10% of any money you have disappearing in fees, exchange rates etc. when looking into moving to another country. Also assume that there will be extra ongoing costs for things like mortgage etc.