You'll need to register as a food business with your local environmental health dept and if you use premises for any processing or storage of meat you'll need to have them approved by the EHO. There are more rules if you sell to someone who will sell the meat on again such as a shop or restaurant. Simplest regarding regulations id to sell direct to the end user which is classed as "farm gate" sales even if you are selling at a market. Best to contact your local EHO and tell them what you plan to do they'll give you all the info you need. Trading standards will also like to know about you so they can check that you're behaving.