24 hours a day, 365 days a year, motorway service areas are required to be open to give you somewhere to eat, drink, refuel and use the toilets. This includes Christmas Day, New Year's Eve and throughout Easter.
During very quiet times (like on Christmas Day) you may only be able to go to the petrol station to use the coffee machine and grab a sandwich, but this still fulfils what is expected of them.
All of the facilities in the main buildings will alter their opening hours over Christmas (not Easter) and it's best to check with them.
This law applies only to official motorway service areas. The rule for official A-road services (services which are signed from an A-road) is that they do not need to be open on Christmas Day, Boxing Day or New Year's Day, and in some cases they don't need to be open between 8pm and 8am. In practice, most large petrol stations will choose to open anyway, but again it's best to check directly with them if you're going to be relying on them.
Finally if you're stuck for something to do over Christmas, may we recommend bringing back the 2020 Zoom quiz favourite: Service Station or Cheese?.
Merry Christmas!