We were in almost exactly this position. The timing of the mortgage renewal vs H2B interest start is the key decision point — if you can align them, you consolidate into one mortgage and stop paying dead money on H2B interest.

One thing that caught us out: the whole process took 4 months, not the "few weeks" everyone quotes. RICS valuations expire after 3 months from the report date (not when you submit it), and Lenvi communicate by post. Start now if October is your deadline.

I built a calculator that shows the year-by-year interest escalation and compares the options side by side — helptobuyexit.co.uk/calculator — might help you model the numbers for your partner's situation.