@irina Hey Irina, I was thinking of joining the AOI too. But yes if you're a member they give you full legal help with contracts and signing up with HMRC (I think).
You don't have to declare until you have got £2000 in the bank from freelance work. You can do it anytime during the year as it's done a year behind. I've just sent my books to my accountant last week for 2017-2018. And I have to pay before January 31st. You get an allowance of £11850 for next year. That means you can earn £11850 before you start paying any tax. Basic rate is 20% up to £34,500. Expenses are tax deductible. Eg art supplies, computer equipment, SVS class fees!
The other thing you have to consider here is National Insurance contributions. Which you have to pay as a sole trader regardless of income tax. It's all a bit daunting to begin with but it's just something you have to deal with if you want to be your own boss.
I'm in an art community called Be Smart About Art run by Susan J Mumford, that has regular meetups and talks from pro artists. Just last week we had a tour of the London Art Fair and we got VIP passes to explore the show the entire week. So they're pretty cool. That's £10 a month membership. Worth every penny. Get's me out of the house.
I'd be happy to meet for a chat and a coffee. I've been in London off and on for about a decade.