"Mark Griffiths reported this unusual event: "Visiting Woodstock, Oxon on Saturday (31st August) we were having lunch when a black and white butterfly, vaguely like a White Admiral, appeared flying around the flower baskets. We saw it a few times, flying up and down the street, including when it was flying around the roof of the Town Hall. Clearly an exotic it must be an escapee from the Blenheim Palace butterfly house. Although I didn't get a close view, when we were about 10 feet away I was able to make a tentative identification. I've been in contact with Blenheim and they have supplied me with a list of species they have. After consultation with my wife (who has better eyesight!) we believe it was a male Blue Diadem butterfly, Hypolimnas misippus."
Here's a link to a photo of the Blue Diadem:
I was not aware of the butterfly house at Blenheim Palace, and it is now highly likely that the observation from 2nd September was also 'an exotic', escaped from the butterfly house.
Thus, the last confirmed sighting from the Upper Thames region was on August 21st.