First you need to do some investigating and some calculating.
I recommend you prove the existing circuit to the garage is suitable and safe for continued use.
You would need to assess the cable route, cable size (CSA), cable distance of route, cable condition and type. To assure this has been correctly and safely designed against the breaker size.
If that is all ok, then by what you have said you have a separate 40A supply to the garage. This would allow for a set-up of a 32A radial circuit for sockets (which will allow for a 7360W demand) and a 6A lighting circuit(1380W demand), The sockets will require RCD protection as will any cable buried within the walls less than 50mm without mechanically protection.
New circuits will require building control notification.
The installation will also require inspection, testing and certificates issued.
Although it is a job that can be done as a DIY project, often the procedures and understanding of inspection/testing/certification are beyond the scope of the average DIYer and the notification fees can be quite tasty.
There are plenty of competent electricians, that can install, self cert and notify the work for you, possibly not a great deal more than the standard notification fees.
PS. I assume this topic has been wrongly posted in Electrics outside of the UK and would have been better placed in the Electrics UK forum.
My comments are based on your Wilts location.
