Hey @ayrshireairsoft - welcome to the community!
Have you looked into how the online sales work?
You'll automatically have access to that information as the back end will create a 'customer profile' from their email address. You'll be about to look them up by name or email and see their totals and sales etc.
The customer sign in is just something that makes it faster for customers making repeat payments etc. - there isn't any additional information that we get 👍🏻
EDIT - for UK GDPR etc make sure you include what information you collect and why. Square has really good GDPR look up tools but you need to make your customers aware of your database use etc.
I don't use any of the info to profile or do anything, so need to let them know that their information is stored. You'll need to let them know what analysis you're doing I think too.
