Change line rental - Impact on Plusnet Broadband

[#1]

I have Plusnet Essentials BB but Phone rental is with BT.
I have a tempting offer from Post Office for line rental only (no contract) and am wondering whether I can keep my PlusNet BB unchanged i.e. would it switch automatically from the BT to the Post Office line?

[#2]

It is likely that the Post Office phone line is LLU  via TalkTalk so your broadband would automatically cease and not only wouldn't you have any broadband you would be hit with a £30 cease fee