Mortgage company will not allow me to let

The plan is to move in with my partner and let this flat out, but Halifax are very reluctant. Their attitude is unhelpful, dismissive, and borderline contemptuous. They don’t even seem interested in converting it to a BTL mortgage. (its a flat, £60K equity, £200K loan, bought 7 years ago, part repayment/part interest only mortgage. Don’t ask…).

I have three shops and 9 flats let out, needless to say letting out another would be no problem for me.

Has anyone else suffered this obstruction to their plans, and what options are there in terms of convincing the mortgage company of the low risk? 

The flat is close by our places of work, and would be convenient for me/us to use as a “crash pad” occasionally. What’s to stop me getting a lodger in, assuming the mortgage deeds don’t prohibit this, and simply not being there very often?

Thanks

Doug