I think it's less about the mortgage provider and more about the tax man, as you'll need to pay tax on the income.
There are also legalities associated with becoming a landlord and the welfare of your tenants (boiler checks, fire alarms etc.). Also change of use. Probably insurance issues too.
http://www.direct.gov.uk/en/HomeAndCommunity/B uyingAndSellingYourHome/LettingYourHome/index. htm
I'd ask the CItizens Advice Bureau, they offer free, confidential advice. At least then you'll know what you're meant to be doing, even if you're not doing it!