Do I need an extra heating channel to use UFH with Wiser?

There is no requirement to have an extra channel on the Wiser Hub to support UFH.

The UFH control strip has a boiler output which can be wired to the boiler to ensure it fires when a UFH channel needs hot water, you can also on commissioning tell the Hub to activate either Channel 1 or 3 (hub type depending) when the UFH requires the boiler to fire.

However, if these channels are already being used for heating you may wish to speak with an installer for help with the installation.

https://wiser.draytoncontrols.co.uk/find-an-installer

So there is no requirement to swap out the Hub.