As the others have said, Harry, no.
There have been several threads on the Community forum but all say the same, and the forum reps have been quite clear on it as well as the retentions/cancellations people.
You can use the 30-day window to tell them you are leaving, but so far as I recall nobody has strongly argued the toss about people with LRS and that they should be able to leave when that expires, once they've been told no.
I have a feeling that Ofcom didn't think of that snare, and that if a group of people raised it with them there may be a ruling in users' favour. Either a deferred leaving, or (less likely) a partial LRS refund. But that's just a hypothesis. It's clearly unfair as it is, and their whole drive is fairness, so there might be a chance.
If you really want to get out in the way you say, then contact Plusnet now and try to get the deferred choice. When they almost certainly refuse then you have the options of either or both complaining to Ofcom and asking for a deadlock letter so you can go to CISAS.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
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Connection - Plusnet
UnLim Fibre (FTTC). Sync 57676/14040kbps @ 600m. - BQM