It is done at headend level. I've explained this a few times on here but no one pays attention.
With FTTC from Openreach all of the cabinets are connected to a nearby headend which is inside an exchange. Not necessarily your closest exchange. A headend takes care of usually 3 or 4 exchange areas.
They have a massive amount of bandwidth to these headends.
Your cabinet may not be connected to your local exchange. In fact there's more chance it isn't. So they don't need mega bandwidth to every exchange, only the FTTC headend ones.
Sorry, but this is not how FTTC works. All the FTTC cabinets are connected to your local exchange, from the local exchange the connection goes to what is called a metro node, this metro node serves many exchanges.