The FAQ states "We don't want our customers wasting time monitoring usage when there's no need to."
Clearly they don't want people to downgrade tariffs, or upgrade to infinity on a limited usage plan, and want to make it difficult for you to consider tariffs when changing suppliers...
BT ADSL customer getting 1.9 Mbps on a new road / new build development
CAB not FTTC enabled, not part of the 66% commercial plan. Not rural - no BDUK funding
(Virgin Media nearby)