This whole tax thing was recently discussed in another thread too.
With respect, it is a nonsense that you can't be self employed if you also derive an income by other means. NHS consultants have been doing it for years. The entire thread is here,
http://www.penturners.co.uk/turning-pens/19565-hobby-business.html?highlight=hmrc and below is a copy of what I wrote at the time.
Just to clarify - you don't register the business with HMRC, you register yourself with HMRC for self assessment, as your activities will be classed as self employment. You only need to register the business if it is something more than just you operating as a sole trader, which in the case of most people here is, realistically is likely to be restricted to maybe a partnership. In that case, both (or all) partners need to register for self assessment AND you need to register the business as a partnership.
Whichever route you go down though, you must register before October in your second financial year of trading in order to avoid being fined if you are found out.
If you ARE making a profit, it's worth considering the partnership route. The onus of responsibility, to all intents and purposes is no different to being a sole trader, but it means that for tax purposes, on paper, your profits are divided amongst the partners, meaning that effectively, if there are two partners you have twice the amount of personal allowances. It's worth considering.
I have to tell you now, that a prominent member of this forum, who for obvious reasons I'm not going to name, has within the last two months been investigated for his pen selling activities going back over several years. He has been presented with a bill for over £5000 which includes various fines for not registering for self assessment.
Markets, fairs, forums, ebay, facebook, etsy, websites etc are all public places. It is easy for HMRC to find us all. They are currently having a purge on ebay I'm told. Tomorrow it may be your craft market. In reference to another thread here in the last day or so, registering for self assessment is also an excellent way to discover just how much you DO need to sell your pens for in order to make a profit.