You have a fault. Your ISP is fixing it. You are not entitled to a downgrade due to not hitting the minimum unless it cannot be fixed.
If they downgrade you that's at their discretion. Brutal as it is you agreed a contract and claimed you were competent to agree it. They are a very small operator so can't as easily take making a loss on you as Plusnet, Virgin Media and your earlier G.fast ISP did.
I believe it would cost them money to move you from your current service to 80/20 as your line needs moving from pod to cabinet.
I think they're doing fine getting an engineer to you to fix the issue on Tuesday. Faults and performance issues happen on FTTP, too. You at least have service and a service that does everything you need it to.
You probably wouldn't be aware of the issue if not for that abominable stats collector. Engineer on the way, fixed on Tuesday, done.