You’ve got a couple of options without touching the motorway: as ART says, A429 out of Stratford > Morton-in-Marsh > Stow-on-the-Wold > Bourton-on-the-Water > Cirencester > Tetbury > Westonbirt > Bath, or the A46 to Evesham > Cheltenham > Cirencester, etc. Probably the first option will be the most picturesque, really, just watch out for Northleach: a rather dodgy steep drop with bends then straight back up again, and it can be lethal; there was a very nasty multiple pile-up the other evening, involving two cars and an artic, two killed and the road closed for nearly twenty hours, one house had a car jack thrown straight through a downstairs window, so just take it very carefully.
Weather should be fine and sunny, so enjoy the drive down, should be a good one. 😀