Dorset in early August
Not entirely sure where you are staying but you could consider Storm or the Guildhall Tavern in Poole. Both great fish places.
Or Matt Follas's Wild Garlic in Beaminster.
Quick lunch near Blandford can be had at the Badger Brewery, the food isn't anything particularly different to usual pub grub but the quality of ingredients is good and so is the beer!
Also in Westbourne, Chez Fred does great fish and chips.