The fact that Fuel Broadband moved customers to Post Office Telecomms means that until the letter came through, the customers had no knowledge their service was being switched.
It may have been "documented here" that Post Office Broadband was "rubbish" as you put it, but whether it was, or wasn't documented, the OP has found himself a user without making it from choice, just circumstance.
I know it is months later, just pointing out that whether it was or wasn't "known" on this forum, the OP was migrated from Fuel Broadband with (probably) short notice.
My phone line was too, and rental more than doubled, so I left them, and took up a fibre deal with Plus.Net (with whom I already had my broadband) and since many ISPs and telecomms firms are charging similar amounts for basic line rental, there seemed no point messing about with a different provider - the cost advantage of Fuel (formerly Primus) was lost, and with any other, the line rental cost would likely be significant.
If you run a business, have a second ISP and backup web hosting...