By virtue of Sky placing a full LLU order it will by its nature cancel the Plusnet server and any telephone billing.
What you need to do is give notice to Plusnet that you are leaving, simplest to request the MAC, Sky may not want it.
Issuing a cancellation notice to Plusnet would result in them placing a cease order (and maybe charging for it), and this can cause issues if the cease is scheduled after the full LLU has started, i.e. this is how some older TalkTalk lines suddenly stopped working in the early days. Openreach got a cease and carried it out without checking that the cease applied to the ISP on the line at that time.