The A34 is normally a good run at that time on a Sunday. There is a very short stretch of average speed cameras at Didcot where they are doing road works. Also look out for speed cameras on the A44 and there are various spots where mobile speed vans sit ;-)
I live on the A44 so if you wanted to carry on up the A34 and then along the A44 I could meet up with you if you wanted?
People from all directions could meet up at the Waitrose in Stratford upon Avon before then going on to Long Marston?