the reason why sunny beach was chosen as a possible location was due to the amount of new supermarkets there that apear to make a good profit but maybe this is not true.
The other fact finding I need to do is maybe this is the reason why nobody has tried it before as the market is to small. we need to answer as many questions now before its to late.
maybe those in the know could advise possible places for such a project for us to investigate
Not that I'm "in the know" but that never stopped me before Look, Sunny Beach is, on the one hand, a seaside beach resort (summer months) and, on the other, a sizeable town in its own right. People live in the town year-round but precious few of them are potential customers for iconic British grocery lines.
You mentioned in your OP that this project has been 2 years in the planning but this is the first time you're learning about the demographics of Bulgarian Black Sea resorts?
7 Red's advice is sound. Take your venture to where the market is - such as Sofia which has year-round resident Brits. But you won't be the first ...
Good luck to you anyway.