My experience mainly outside the South East is virtually every half decent UK bakery will make barmcakes and sandwiches to order, for about the same or if they are more it's only by a little - say 10-40p. Yet you'll usually get vastly more fillings, so they work out much cheaper, along with scope for customisation.

The chain bakeries like Gregg's etc just offer the same sort of carefully rationed pre-pack choices as supermarkets.