Or a slightly more cavalier approach might say dont tell your lender a goddamn thing - worst case you get a moaning letter if they ever find out.........ultra worst case thats never happened in the history of time is that your in breach of your terms and they can call the debt in.
Tell them and chances are they'll charge you a fee and load your rate whilst demanding a billion and one forms.
Second piece of advice.......instead of letting the property have you thought about stapling your scrotum to a really disgruntled porcupine? The reason I ask is because being a landlord is about as much fun.
Get a local chav to firebomb the place instead.
Yes, informing the lender can cause issues, but if you let the property and the house burns down, if you haven't got the correct landlord insurance they will refuse to pay out. If you have got the correct insurance, the first they will ask for will be proof that the mortgage was a BTL or at the very least you had Consent To Let. If you have neither, you'll have a very expensive pile of rubble on your hands....
Pretty uninsured I am then!