I've been with PN for ten years now, and just over a year on FTTC. 80/20 unlimited.
I've no complaints about speed, my connection syncs at about 47/9 and I never had any congestion issues.
I'm pretty sure they use BTs back haul.
Support is UK based, but on complicated faults you can tend to keep going over things, as you get various support agents dealing with the issue, I suspect this is true of many bigger ISPs.
Their forums are also a good place for support with many active staff members.
It's also possible to reduce your monthly payments via refferals, all the time someone you refferred is with Plusnet you will get a discount off your bill. If you refer enough people they will even pay you.
I've recently moved our work broadband over to business fibre with them, and I wouldn't have done this if I wasn't confident with them.