The Rushden Cavalcade of Historical Transport and Country Show takes place in a few days' time, over the coming May Day Bank Holiday weekend, and has for many years had an excellent real ale bar.
*As I write this*, the link from their home page
shows last year's beer list
and a further link on their beer list page
shows last year's cider list,
but I expect there will be a very good selection there this year, as usual.