Many garden centres sell food, but most seem to be overpriced and uninspired. Stewarts is an exception to the rule, with decent food (excellent cakes!) very competitively priced. The "afternoon tea for two" is a particular bargain consisting of a plate of 8 little sandwiches (4 varieties), plus a second plate with two small cakes or similar (recently they were fresh fruit tarts), shortbreads and four scones with jam and clotted cream. You can choose to have tea or coffee (or one of each, as it's for two people) - and all for under a tenner!
Stewarts also do hot food - the quiches looked good and were reasonably priced with the option to add a self service salad plate. And the large cakes/gateaux are also a good deal, with a wide selection of calorie laden cakes!
There's plenty of seating areas, including some outside tables on nice days, and free wi-fi if you are not really into gardening.