There must be lots of people in your position who have an interest only mortgage – unable to pay off the principal sum at the end of the 25 year term.
I would guess the loan amount to be around £50-60k and therefore you have around £140-150k equity in the property.
Given the level of equity – I would seek advice from a mortgage broker. I would be very surprised if they could not find you a lender who is willing to extend the interest only mortgage; given that there is no risk (to them) in such a loan.