There arrears to be two different ways of using the reset button on the BT Home Hub 5A in version 19.07.1 (I don't know if this applies to all devices or just this device).
Holding reset down while powering up until the LED starts flashing (green on this device) and the device goes into failsafe mode (with rapid red LED flashing on this device).
Holding reset down longer while powering up until the LED flashing changes to a different frequency causes an immediate device reset to uninitialized state.
The problem I have found is that the second method of resetting does work. The first method (failsafe mode) followed by the firstboot command has no effect.
The command response to the firstboot command is:
root@(none):~# firstboot
This will erase all settings and remove any installed packages. Are you sure? [N/y]
y
/dev/ubi0_2 is not mounted
/dev/ubi0_2 will be erased on next mount
writing /dev/ubi0_2 failed: Operation not permitted
On subsequent reboot or power cycle the original configuration is still present and there is nothing in the system log to indicate it tries to erase flash partition ubi0_2.