A site we really like is called Diamond Caravan & Camping Park in Bletchingdon near Oxford. It's a small site, very clean and well maintained. I left a review on here, and I would recommend it.
Colin D,s recommendation is spot on,Diamond Park is almost at centre of triangle of OPs locations.Caranan clubs site at Blaydon is close and Hardwick Park at Standlake is not bad but nearer to Witney.