Will UKAR let me switch to BTL?

I have a 3-bed in central Birmingham that has a UKAR mortgage of around £250,000 set at 0.5% above the base rate apparently in perpetuity.

I’ve weathered the storm for the last ten years and the value of the property has risen. This is the only mortgage and the only property I have.

However, for personal reasons I now need to move out of the house. My financial circumstances have changed since taking out a huge self-cert many years ago…

Under my current mortgage conditions, it seems crazy to rock the boat. I’d like to take on some tenants, and ask UKAR if I can switch to a BTL mortgage. There’s enough potential in the house for income that it wouldn’t be a problem to do this at all, in terms of making the mortgage payments.

But having read the various horror stories about how UKAR operate I’m scared that if I ask, they’ll either say “no” and then watch closely to see what I do, or that they’ll say “yes” and then wait eg a year and then retract their permission. (That’s what I’d do, if i was wanted to clear my book of mortgages!)

I suppose I could just proceed without asking their permission, which would result in a decent amount of income, which I’d then declare to HMRC as I have no intention of breaking the law re tax! If I were to do that, would UKAR be able to find out from HMRC that I was declaring an income from property, or would that information be private?

Does anyone have any experience of how they might operate in this circumstance? Am I being too cautious by not asking, or am I right to assume the worst?

Gary