You may be confusing the early termination charge for breaking the 18 month minimum term you are 8 months into, with the broadband Cease
charge for cancelling service without migration.
I can give you a link later if you want to the relevant section of the price list. Using my iPad at the moment.
If you weren't made aware of the new minimum term when you downgraded then you have grounds for challenging it. The best route is via the Community Forums, that you can get to from the Plusnet Home Page.
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.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59999/14372kbps @ 600m. - BQM