Really rural exchanges with no 21CN may well have a rate limited portion of an E3 - a 34 Mb circuit - that fraction being as low as 10 Mb for data services via BT Wholesale. PPPoA baby!
The PSTN is a different matter. Each call gets 64 kbps so for a small exchange a simple E1 - 2 Mbps providing 32 channels - will be ample.
The handover exchanges are relatively fibre rich. You can shove terabits down a single fibre, so it's whatever space, power and cooling is available in the exchange to hold the transmission kit.
The Salisbury exchange for instance is going all FTTP. One operator alone has purchased 80 Gbps of Cablelink capacity which they'll presumably then connect to a 100 GbE or a pair of 40 GbE links to get to their own network.
You can have a wild guess at the bandwidth an exchange might need by its premises count and the number of businesses that might be packing leased lines. Allow, generously, 2 Mbps per residential property.