I have no doubt that Plusnet can delay the installation date but be aware that the actual date is determined by Openreach engineer availability, which rarely exactly matches the requested date.
The other thing to consider is that unless you are moving from a cable supplier you should not cancel with your existing ISP, you should leave it to Plusnet to liaise with your existing ISP as they will be taking over your service, not cancelling the line. If you cancel yourself and Openreach are not available that day you run the risk of being left without phone or internet and potentially losing your phone number. There's also the chance that you will end up with a new line charge and if the fibre cabinet is full with a waiting list, no fibre broadband availability
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