Letters from mortgage provider requesting info about how I plan to pay off mortgage

[billygoat24]

I have a 20yr mortgage which ends Nov 2027. Originally it was repayment mortgage, but I switched to interest only early on. I have no arrears and have never had any arrears. I received a letter and phone call requesting I inform them how I intend to pay off the mortgage at the end of the term. I received a couple of letters like this periodically several years ago, which I ignored. I have no repayment vehicle in place, but I'm confident I will either be able to repay the full amount or sell the property (although I have no way of proving this right now). I owe approx. 290K, the property is worth approx. 700K. In the event I do not have the funds to pay off the mortgage, I plan to make the decision to sell 2 years before the mortgage ends. What legal obligation do I have to provide evidence of a repayment vehicle to my mortgage provider at this stage? Also, what action can they take if I don't?