Echoing what others have said, Bolly. I don't think an underbed enclosure would be the best option. And I definitely wouldn't use a combined/uv bulb in such a confined space. In fact with any heat bulb, it would pose a fire risk. You must have the space to adequately house the tortoise - indoors and out.
If not, I'm sorry but it may not be the right time to be an owner. xx