BT Smart Hub 2 and Apple Time capsule & Airport express etc

Is there a benefit in retaining these with Smart Hub 2 in Bridging mode to support AirPlay streaming of music?

Yes, If you want to continue to back up your Mac(s) to the Time Capsule and use the AirPort Express for AirPlay. Note......it is the Time Capsule and AirPort Express that will be set up in Bridge Mode......not the Smart Hub 2.

I am hopeful Smart Hub 2 will better tackle my blackspot with the Wifi Disc?

Good luck! Unfortunately, no one on a user-to-user support forum will be able to accurately predict how well the Smart Hub 2 will perform in your particular situation.

The Time Capsule must connect to your BT Hub network using a permanent wired Ethernet cable connection.

Ideally, it would connect to your main router......the BT Hub......but it could also connect to an extender or repeater......IF.....that device has an available Ethernet port that provides a network connection.

By default, the Time Capsule provides a wireless network signal, but if you will be using the wireless provided by the BT Hub to back up to the Time Capsule.....and you don't need another WiFi network.....then you could turn off the WiFi on the Time Capsule.

If you turn off the WiFi on the Time Capsule, the AirPort Express will need to be reset and reconfigured to join the BT wireless network.