To transfer out a domain (which is not a .co.uk) you need to supply your client with an EPP code which they then give to their new registrar. When you click the Get EPP Code button in WHMCS you get the following message:

The client will now receive a valid EPP code for the relevant domain. This EPP code will expire it is either valid for 24hr, 48hr or 7 days but I am not sure which.