No leave everything in the hands of Plusnet.
If you tell your existing supplier(s) that you are departing, they might fix the end date and raise service cease orders. That will cause Plusnet's take over orders to be cancelled until after your existing supplier's service termination has occurred. You then have a marked risk of being without a phone and broadband service until a replacement service can be provided. There is also a marked risk of losing your existing telephone number.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
