They won't give you promises in writing. If you don't want to "upgrade" then don't. You can't really lose out though. All calls with BT sales staff are recorded. If there's any dispute then they check the call back. You can also ask the sales staff to put a note on your account stating that you've been promised a free regrade back to ADSL.
You've escalated to the ombudsmen that they won't let you offer FTTC. Now they will, you want specific personal guarantees in writing. I'm so glad I no longer work in customer services. I have a feeling you would be a nightmare to deal with
It can't do any harm to try it. You may get 5-6Mb. I've seen similar from slightly longer lines.