I haven't registered (yet?) - but I make very little - £50 a month on a good month. I think you have 3 months to register once starting as self-employed. I'm not the one to ask though as am burying my head in the sand as to register would mean I am actually taking the plunge. I think it's a matter of paperwork rather than anything else when you're a small business. Try posting in employment to get real advice.
Flubba - what kind of fabric things do you sell? I'm teaching myself to sew again & am bored with drawstring bags! I need to broaden my horizons somewhat. I am just beginning to make reusable snack bags & sand which wraps too.
Tax office needs to know you are self employed, as others have said. I got my PLI with Market Insure, and I think it was around the £50 mark. Needs to ensure you against a problem caused by workmanship etc.
Grockle I make all sorts of things from bunting to little named cushions to simple clothes for my DCs to key rings. I've just made an owl-pyjama case for my DD's birthday which I love. I have lots of free tutorials on my blog which you're welcome to check out. If you search under tutorials and then scroll back for older entries too, you can see all of them there.