Hi shelle,
This is commonly known as a 'consent to let'. Very very common for people to do this. Various lenders have different views on this, some will do it provided it meets certain criteria, and some will do it no matter what the loan to value / rental will be. Im pretty sure that Northern rock are one of the flexible ones as they have a lot of customers in deep water thanks to their previous lending record.
I rent out a property myself as I have moved in with my partner. My lender isnt aware that I do it, as they have certain criterias which I do not meet with them to change it over, i.e. Loan to value and rental income. So I basically do it behind their back. If you keep up repayments and give them no reason to question you, then you should be fine. Worst case scenario, they could force you to re-mortgage in extreme circumstances.
As I said, Northern Rock tend to be more flexible. Give them a call and give them the hypothetical situation of 'travelling' and that you need to have a consent to let the property. I think they charge a small fee for this. Give them a rental figure of somewhere near what your mortgage payment is on interest only and they should be fine with this.
If they do not agree, you can potentially proceed with renting it out, but at your own risk. The chances of them finding out are tiny, and a lot of people rent out properties without consent!
Give them a call, then post back how you got on. Remember to use 'hypothetically'