Presumably you would need to comply with a whole load of trading standards regulations, like cleanliness, hand washing facilities and so on
Don't forget to work out the costs of running refrigeration
Also, consider how you are going to go about restocking your shop. Lugging stuff down the towpath for your own consumption is hard enough, what about multiplying it to cover your customers needs
And then there is the VAT man
Richard
(My in laws ran a shop similar to what you are proposing. It was bloody hard work and that was with good regular customers buying papers and using the post office)