Having looked at your installation guide am I right in thinking that a registered electrician is required for installation so as to obtain a warranty ?

Anyone can install one of these saunas, indeed they are designed to be quick build and any DIY'er can have the body assembled within 20 minutes. If there is a plug socket nearby, they simply plugin.

UK law states that any new electric supply (feed from the mains board or ring main) is installed by a 'Part P' registered electrician. This will more than likely be part of any home insurance also. 

The manual refers to this, but for the actual build of the sauna and 'plugging in', anyone can undertake this