In essence not a 'licence', but the Food Safety (General Food Hygiene) Regulations 1995 covers sale of foodstuffs generally in UK, particularly England. You must go and talk to your Council re the required records, equipment to store, hazard analysis etc etc. Also, the local environmental health officers will be able to point you in right direction re food handling. And check your planning regulations - do they exclude food retail from your premises?
Stock control, staff training - suggest a minimum is at least basic food hygiene certification - suppliers details for the range of goods - seperation of organic from non-organic and the like - disposal of unsold stock - all this has an impact but maybe before just setting up the faclity, do a good deal of research on your current customers and determine if they want the staples - fruit veg, dairy etc, or if they will spend on luxury / impluse buys - liqueuers, preserves, ice creams etc. This will have an effect on the type of goods tyou stock of course - are there any similar farm shops close by (that you can visit to check their operation) that will dilute the potential market for you, or can you be the initial business in the area?
You may want to contact the guys at FARMA -
http://www.farma.org.uk/ Gavin and Sue will be able to give you some suggestions about a farm shop operation to extend your business - and Big Barn
http://www.bigbarn.co.uk/ would love to list you as a faciliity for promotion amongst the many options.
Hope this of help - any promotin of local reatil is a benefit to the community and there is a good opportunity here for you if it all appeals.
Good luck!