Instead of HMRC, who don't offer advice on business matters at all, I'd contact trading standards. They actually very helpful IF you can contact them. They now use a centralised system, so a call to your local council office might get you through to somebody.
Work on the principle that if anyone gets cross with you, it will be them - so they will happily advise you because prevention is better than enforcement if there's a problem. From the fire perspective - they were very heavy on clothing that went up in flames - the costume fiasco recently - where kids clothes were not clothes, they were toys and dangerous.
Public liability insurance protects you from claims from the public. If somebody puts on your clothing and the factory didn't wash out the chemicals and somebody comes out in blisters - it's your fault. If a pin is accidentally left in, and they blind themselves on it, it's your fault. If they spill wine on a red dress and the dye comes out and stains an expensive leather suite, it's your fault. Insurance might cost, but only you can decide if it's worth it. In my business, I have insurance that covers me when working in customers premises - did I hang that light properly? Did I really leave that cable across a main through route where somebody fell over it. I sell items by mail order too, and I have decided that what I sell is unlikely to cause problems, because my customers are mainly professionals. However, what if somebody got electrocuted by a poorly made power supply?
Register with HMRC as a self-employed person (even if you have a PAYE job too), and you get a UTR number. Open a bank account for the business to keep it separate and HMRC friendly for end of year purposes. VAT is probably a non-starter as clothing doesn't attract VAT, does it?
It could well be worth a chat with an accountant. Most smaller firms will give you a 30 minute chat for free. Even if you don't have massive turnover, you may find they will guarantee to save you more money than they charge - mine certainly has been useful over the years, even though he really only prepares my end of year accounts and does my self-assessment form, There are entries in boxes I don't understand that reduce my tax liability. I'd have left these empty. They will advise on the proportion of your utility bills you can claim for. I use lots of electricity, so 50% of my electricity is allotted to business, but just 33% of the gas. HMRC won't advise you on things like this - they just want you to be reasonable, and you decide what reasonable means!
You can do this bookkeeping yourself, but if you are like me, you don't actually understand the implications of claiming for this, but not claiming for that. My accountant takes my receipts and invoices I collect (that I have already filtered, removing those that I don;t think are business related) and throws out even more - but sometimes reminding me some are missing. I repainted my studio, replaced the carpets after I had an accident with a cup of coffee that got thrown across the room. I put in some new lighting at the same time. I'd mentioned the accident and what I annoyingly had to do - the cost of the repairs comes off my profit, meaning I pay less tax.
One thing I would absolutely do though, is check your house insurance if you work from home. I changed mine to a new policy that covered my working from home business. We bought this house because it had been extended and had a brilliant space for my studio - safe and secure and all alarmed. Councils are generally happy for people to work from home - unless you have customers calling. I don't and this is insurance friendly. If you don't tell the insurers, it could be bad in the case of a claim.