Time Capsule

Can you clarify what you mean when you say that you are trying to configure the TC to "join" the network?

Are you trying to connect the Time Capsule using a wired Ethernet connection to the BT HomeHub? (Recommended)

Or, are you trying to connect the Time Capsule to the BT network using wireless only?

Thanks for the clarification. Do you want the Time Capsule to also provide a wireless network, or do you plan to use it for backups only?

Are you able to open AirPort Utility and click Manual Setup and access the settings on the Time Capsule?

can open AirPort Utility and but if I click Manual Setup it switches network and I lose internet connection.

This is normal. The Time Capsule is broadcasting a default wireless network with a name like Apple Network xxxxx. If you want to configure the Time Capsule using wireless, you must switch networks and log on to that network so AirPort Utility can communicate with the Time Capsule for the configuration.

If you don't want to use wireless, and you have a spare Ethernet cable handy, connect it from your computer directly to one of the LAN <-> ports on the Time Capsule.

Make sure that the Ethernet cable from the BT device connects to the WAN port (circle icon) on the Time Capsule

Open AirPort Utiliity and click Continue to follow the guided setup to "create a wireless network". If you prefer, I can give you the steps for a manual configuration.

But once I go down that route I won't be able to use the internet to get back to you.

Once you have the Time Capsule configured with your new wireless network.....let's call it "Gary" for example, you will look for this network from your computer and log on using the password that you established for the wireless network. Then you will be connected to the Internet again.

Or, if things go wrong during the setup, all you have to do is look for your BT wireless network and log on to establish an Internet connection that way.

Temporarily.....while you configure the Time Capsule.....you are not connected to the Internet. Once you have it configured, you will be able to connect to the Time Capsule network or your BT network....whichever you choose.