Don't know how long you've known your partner, but are you 100% sure this relationship will never break up, because unless you are pretty certain about it, you could be very foolish to leave yourself without a fall-back property in which to live, especially if property prices keep rising. So I'd keep the house and rent it out so you still own a rising investment..
If I were in your shoes I'd go for a repayment mortgage so you know every year how much you are paying off. If you go for interest only and are relying on some kind of with profits endowment property to fund it, if the stock market doesn't perform you could be left with a large deficit at the end of the loan period.