What is this drill guide/template used when fixing electrical sockets called and where do I buy one?
It's called a pattress jig.

B&amp;Q stock them in the UK.  However they can only be purchased from a physical store not online, the ean code is 03619377 and the cost is £7.68. (A B&amp;Q store can order one if needed based on the ean code.)

There is also a seller on eBay UK.