The Wincanton straight line mark shown below is known used from 1801 to 1807. The entire, to Birmingham, is undated apart from "1st July".
The entire has two postal markings, an "8" crossed out, and a stylised "10" (the spiral). From 1801 to March 1805 8d was the postal rate for 120-170 miles and 10d was the rate for 230-300 miles (on 12th March 1805 the postal rates went up so that 8d was 80-120 miles and 10d was 170-230 miles).
Wincanton to Birmingham direct is about 125 miles while going via London is about 245 miles. This fits with the 1801 rates as 8d would be the rate for going direct while 10d would be the rate for going via London.
This would date the entire to be from July 1801 to July 1804.