I get leggings and sports bras from SimplyBe, and a long vest from Yours Clothing (not a specific sports one just a plain cotton vest that is long enough to cover my tummy). Both deliver pretty quickly and returns are easy if stuff doesn't fit.
If ordering from Yours Clothing if you browse whilst logged in but dont order anything then they often email a 10% discount code within a day or so.
SimplyBe also do trainers from various brands but to be honest if the dog walking ones fit and are comfy then I'd just shake off any mud and wear those.
If you are after somewhere you can pop in person then try supermarkets - Asda, Tesco and Sainsbury's are good for plus size.