You have an electrician who has carried out the first fix (fitting the back box and running the cable). When he comes back he carries out the second fix (attaching the faceplate) and then tests to ensure it safe for you to use. Wait until he finishes the job.
Look at the switch and load on the quickwire website This is a 4 terminals jb.
This is the link to the page. https://www.quickwire.co.uk/product/switch-load-16a/
Hope it helps.