If you trade with out setting up a limited company then you are a sole trader in the eyes of the law. This is no problem and makes admin very simple as there isnt any apart from pay tour income tax. The down side is if someone trips over your kit bag or you damage something in your clients premises they will sue you as an individual and if you loose they can peruse you for compensation. In an extreme case this may mean you loose your house to pay a personal injury claim (or perhaps even just the legal bill to defend yourself).
If you register as a limited liability company there is more paperwork, but not much. You will need to file annual accounts which are very basic for a small business, you will need to file a corporation tax return (form CT600) and pay tax on your companies profit and you will also need to have employers liability insurance as you will be employed by the company as a director. You also need a registered address and a company sectary. These last two could be your spouse and your home address but they dont have to be. For about £100 you can employ a third party as company sectary and registered address. You would also be well advised to have public liability insurance. Having done all this if you have a mis-hap the claimant would pursue the company as a legal entity not you. If the company was found to be negligent the claimant can only pursue the company for the assets it holds i.e. not your house or other possessions. In this instance most small limited companies would declare bankruptcy bringing an end to the claim, this is why as a responsible director you would have public liability insurance. If a company is declared bankrupt the director can set up a new company the next day if they wish, you may be GB photos Ltd one day and Photos GB Ltd the next. If you are personally declared bankrupt you cannot be a company director and you would have credit trouble for ever more.
In summary for one simple low risk job be a sole trader, for an ongoing business venture open a limited company.
I hope this helps.
P.S. this is for information only any action you take is your responsibility, please dont sue me. I have to put that in for legal reasons!