Tim, The main reason local section get tee shirts etc done themselves is any money generated stays in the section and helps fund the local section and their rally costs. If you centeralise it then the local sections loose out, unless the main club is going to finance local section rallies.
Two or 3 years ago the East Midlands section had some polo shirts done with their section name on them and they were done local somewhere near Chesterfield. The polo shirts are a good quality and the cost at the time was £17 per shirt. Compare that to what the regalia prices are currently for a polo shirt.