For the most part, this item is fairly typical of mail from the United States to the U.K. in 1865. The treaty still called for 24 cents per half ounce, so a 24 cent stamp is used to show payment of that rate. It entered the mail at Street Road, PA and then was placed in a mail bag to cross the sea in Philadelphia. It was taken out of that mailbag in London and delivered.
The cool thing about this cover is the adhesive on the top left. Westtown school and farm was around a mile away from the rail station, where the nearest post office was established. At that time, it was illegal to have a private mail carrying service on any road that was designated a 'post road.' However, there was no such service to the school and farm. So, a service was provided for 2 cents to carry mail to the station. The adhesive (or stamp) at the top left indicated payment for this service.
Westtown school is a Quaker school that still takes boarding students and the section of the West Chester and Philadelphia Railroad around the old station now runs a historical/tourist train through this area.
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