Here are a number of earlier Ilminster covers, starting with one from London in February 1744. As usual with older letters it is about money.
This next entire is from South Petherton via Ilminster to Nether Stowey near Bridgwater in May 1787, with a "141 ILMIN / STER" mileage mark (the first type issued). The letter is marked "X Post" to make sure it goes cross post and not via London (which would have cost significantly more).
The printed letter below from the Legacy Duty Office to Buckland St Mary in November 1825 was redirected to Bideford, then Plymouth or Tavistock and finally to Ilminster where it received an "ILMINSTER / 143" circular mileage mark.
Here is a clearer example of the "ILMINSTER / 143" circular mileage mark used in February 1829 on a wrapper to London. It was originally rated at 1/8d (double rate) before being uprated to triple rate, 2/6d, with the "1/8" crossed out and overstamped with an Inspector's Crown handstamp.
Here is another example of the postage charge being uprated by an inspector, also from 1829.
The letter below to Cirencester in 1829 has an example of the circular "ILMINSTER" handstamp with the mileage erased. It was charged 10d postage (the circular curly squiggle).
In November 1839 the entire below was from Winchester to 'Near Andover' but was missent to Ilminster and received the fairly rare "Missent to / Ilminster" handstamp.
And finally the entire below from Ilminster to Wells in June 1839 received a black example of the "ILMINSTER" single arc dated handstamp. The handstamp is known later in blue and in red.