Same question, same answer, so I've just copied and pasted:
It is a condition of most standard mortgages that only the mortgagee and family live in the property and it is not let out. Read your mortgage contract - it will also say you need to ask their permission to rent it out.
This is simply because if you default on your mortgage it is very much harder to repossess the property if there are tenants living in it. Yes, you have a guarantor but this does not mean the mortgage company can't be in a position to repossess.
The mortgage company is in the right in the instance. If you are not happy you can always go to another mortgage company.
Remember you will also need to inform your insurance company that the house is rented out - your insurance will probably go up as well.