Hey nyawton,
I don't know too much about laws in UK but I am quite sure you have to pay taxes... isn't that a given pretty much anywhere but tax havens? But if it is an extremely small store and you don't make money from a regular job you might come in below the personal exemption. You might want to check out
http://www.smallbusiness.co.uk/ It looks like you could probably find out pretty much everything you need to know there!
Hope this helps!