I had an "interesting" experience. It comes under the title of "What's the worst that could happen...?"
My order was through Plusnet, and (at first) went fine. When I tried to put the install back by a couple of days, the order got b0rked on BT's side (Wholesale or Openreach), and got stuck - couldn't be removed, fulfilled, or replaced. So we couldn't make *any* order at that point.
At that point it went through all manner of escalations at BT's side (I guess a "technical team" or two), BT still couldn't get rid of the duff order.
Eventually, they (BT) decided to just cancel my existing broadband, which just disappeared one night. We (ourselves & Plusnet) hadn't agreed to that step, mind...
It took 10 weeks from start to finish.