BT offer the Stay Fast Guarantee which imo is better than the usual MGAL (Minimum Guaranteed Access Line speed) you get with FTTC.
MGAL is the minimum guarantee sync that ISP's usually quote when signing up and refers to the sync speed.
BT's Stay Fast Guarantee measures throughput to the hub.
They also give you £20 if they can't fix any issues that mean the line doesn't meet the guarantee.
As pointed out above residential broadband is a contended service.
For an ISP to offer guaranteed throughput is a good thing.
If you want a higher guaranteed minimum there are plenty of very expensive business/leased lines with SLA's that will give you this.
Their Stay Fast Guarantee on their 1gig FTTP (1000/115) is 455Mb.
That's a very high guarantee for a residential ISP.
It's higher than the rates that OpenReach guarantee for the service.
Check their other FTTP Stay Fast Guarantees
https://www.bt.com/help/broadband/full-fibre/what-is-bt-s-stay-fast-guarantee-for-full-fibreIt's something to be applauded imo.