A UTH will do nothing for your ambient temps, whether you live in a house or an igloo. You will need to heat your house to keep your ambient temps at a reasonable level.
As Jason stated, you will need a thermostat to run any heating element.
I would ask the Boaphile about RHP sizing. The Boaphile will take into account the species of animal, the size of the cage and them temp in your igloo and make an educated recommendation.
I would go with the one that the Boaphile can install while fabricating the cage. That way when it is delivered to your house, all you have to do is plug and play.