At this time of year the mail centres and delivery offices depend on Royal Mail managers from all parts of the business giving up 2 weeks away from their day jobs, to help with the mail. They also rely on temporary staff, I'm guessing that it's just not practical for social distancing etc as the work areas wouldn't take the vast numbers of people required to keep the mail sorted and moving around. Many Christmas cards get rejected from the machines due to odd size, no postcode, and illegible address due to coloured envelopes and shiny pens !