My question about FTTC and Fast is that as Fast is a wholesale customer of BT's, wouldn't the network characteristics be the same for BT Infinity and Fast customers?
No, not at all.
Think of both FTTC and ADSL that use OpenReach fibre and BTwholesale 21CN connectivity in the same way as the old "dial up", where you could dial into any ISP you had an account with.
The fact BTwholesale provided the telephone lines to connect your modem to the selected ISPs modem didn't affect the internet speeds/connection.
How the internet service performed by that ISP was down to how the ISP network was set up and who they contracted with for connectivity.
Well OpenReach fibre and BTwholesale 21CN networks connect you into the ISP, which could be BT Retail and their "BT Infinity" product, or it could be "BT Broadband" or it could be FAST or ADSL24, or IDNet etc.
OpenReach and BTWholesale provide a fast connection into the ISP - but not to the internet - if there wasn't a difference then the Competition Commission / Ofcom would want to know as the BT group would have a (complete) monopoly.
I feel there should be some simple diagrams of this, the ThinkBroadband "how it works" is good for how DSL works but doesn't show the separation of backhaul and ISP/Internet traffic.
pro - on THFB - sync about 17.2mbps - BQM