I would say that some sort of public and product liability insurance is essential if you’re selling your work to the public. Most craft fairs will insist on it and will want to see your paperwork. HMRC are very helpful, and the government website has a very useful step by step guide to setting up as a sole trader and when you need to register: Set up as a sole trader: step by step - GOV.UK
There’s a thread about insurance somewhere in this forum, let me see whether I can find it…
ETA: yup, here you go: Insurance recommendations?
There is also loads of advice on here about selling at fairs, setting up your shop and anything else you might need to know!