Breeding any kind of animal is usually a labor of love (that means there's no profit involved). Unless you have a really large scale breeding facility with a lot of wholesalers ready to buy your animals in large quantities, you'll be extremely lucky if you can break even. When you consider all the cost of initial equipment, electricity, food, supplies, vet bills, time, etc, it get's very expensive. My breeding projects barely cover the cost of food and supplies alone. I've only recently made back the money I originally spent on my breeders. If I am able to continue successfully, I should be able to get to the point where it will also cover my utility expenses and
maybe a very tiny portion of my time. At the end of every season I seriously reconsider whether to stop breeding completely. But I do love the excitement of seeing new eggs being laid and then the hatchlings growing up.

But Bearded Dragons are much easier to sell than RES are, and they are a lot easier for beginners to care for too.
Anyway, it sounds like you may have a pretty good relationship going with your local pet store and if you do breed your turtles, you could probably play a big role in making sure the potential buyers are aware of what is required (you could write up your own caresheet for the pet store to display and hand out).
But breeding RES as a hobby can be an irresponsible act if you do not already have a good market for them. Around here, they are sold by the bucket full in flea markets or in dirty shops and overcrowded conditions. And the people who sell them know next to nothing about how to take care of them so the people who wind up buying them have no idea what they're getting into. Inevitably the poor turtle ends up in a terrible living condition or gets sick and dies. Not a good life for such a wonderful little animal.
Turtles do breed and in the wild they thrive very well on their own. Too well actually as they have been illegally introduced into many areas and have outcompeted the local fauna. So there is really no need for humans to be breeding them anymore, at least not in the numbers they are being bred at now. That's here in the US and primarily with the RES. The situation sounds like it may be better in the UK.