Re: Mortgage Repayment
I agree with others in that I see no real benefit for your partner in this arrangement. It seems to be all upside for you and none for her. Further, she would carry the significant risks too.
It would make more sense to me if the savings on your mortgage repayments were to be invested into increasing the deposit on the mortgage for the new property for both your benefits. That doesn't appear to be the idea here. In fact, it seems you are intending to own your new property as tenants in common rather than joint tenants.
If you and her agree to do this, my view is that any loan should be recorded in writing, making it clear what the trigger(s) for repayment is(/are). This should mitigate risk for her if any unforeseen events occur.
I'm not a tax adviser but cannot foresee any income tax consequences if it is an interest free loan. If it is interest bearing, the interest would be taxable income in your partner's hands. I believe there would also be inheritance tax considerations too, depending on the value of her assets. I anticipate that her estate would be an excepted estate currently given the deposits intended to be paid on new property, unless she owns other significant assets.
Generally for me, loaning money to significant others is a no go beacause it risks jeopardising the relationship, but each to their own...
I recommend you each think this through carefully and seek appropriate advice.
See here for a useful overview of issues to consider: [www.bdo.co.uk].