Possibly more could go at weekends but that would run up against Network Rail's need to maintain the railway.
In more ways than one. Not only are some lines / routes closed or with reduced capacity (for example the MML is a 2 track railway south of Radlett today, and the Hendon freight lines are also closed)... but the locos and drivers that work commercial freight in the week are working engineering trains at the weekend. The latter is one reason why there won’t be a wholesale shift of engineering works out of weekends post Covid.