Residential or Buy to Let?

I am looking to buy my first home in the next few months. I have unusual circumstances though. I work abroad and I want to buy a place for myself however if I get sent away for longer periods due to promotion etc I would not want to leave the home vacant. Will my residential mortgage lender permit me let the home out temporarily or do you have to convert/re mortgage to buy to let? I have friends who have done this with their homes however I worry that the mortgage I get may have something written into it that the home can absolutely not be let under any circumstance. I will be working all over the world for the next three years however I do not want to wait to buy as I have my deposit and want to buy now. As I understand it should not be necessary to go down the ;buy to let' route for this purpose?
Any suggestions, advice and help would be greatly appreciated. Also, would it be best to work with a broker in this instance?
Thanks