I have a bit of a problem regarding BT Broadband, long story short but I accidently clicked to extend my Broadband for another 12 months in order to get BT Sport but I actually wanted to do extend it for my broadband on my second line.

Will I have to pay early cancellation fees or will BT allow me switch to the other line/ allow me a cooling off period?

Any help would be greatly appreciated

If you have two lines with them I would like to think they'd help you out here. Just give them a call and ask.