kids Apple watch cellular

Bought my son the apple watch SE cellular - he doesnt have mobile phone but does have a phone number (used on a previous kids watch).

i’m in UK and cannot find a monthly SIM package that works with the apple watch but also allows him to keep his mobile number. They all want a mobile plan but the whole point in getting the apple watch was to hold off getting a mobile for a few more years!

Feels like the apple watch has now been a wasted purchase and we should have stuck with the other kids watches!!