Like someone else has mentioned you're more likely to find them in corner shops and in the discount stores like Home Bargains and B&M.
But in my opinion they're probably not readily available because they're just not very popular. Gummy Bears are a bit boring and bland compared to the other sweets available.
Growing up I'd sometimes get a 10p mix up from the corner shop after school and there's no way I'd have wasted a penny on a gummy bear when I could have gotten a giant strawberry, a ufo, a black jack, a fruit salad, a fried egg, or best of all a foam shrimp/banana.