Setting up as a business

Welcome and congrats on your book. Do you mean set yourself up as a business like becoming an LLC? Most authors don't even worry about that. Most authors begin building their brand from the first book and you are automatically a business once you publish because you're selling to the public. Some authors use LLCs but it definitely isn't necessary and even could cause tax issues. If that's what you mean, then look up the guidelines and laws in your area of the UK about setting up an LLC and taxes, etc.

If you were just speaking generally, then to begin building your brand, you need to have everything a professional author should such as a mailing list, website, etc.

I wouldn't waste time running ads or doing too much promo for one book. Right now, focus on publishing more work because the power will be in your back list. I'm not saying don't promote the book but don't go out there spending thousands of dollars on ads or something because most likely you won't make the money you spend back unless your book becomes a runaway hit. You can't gain much traction with one book so concentrate on having a website, mailing list as I said, stuff like that. Not just short term promo for the book. When you get to about three books (especially if this is a series), then you can start marketing more seriously but at this point you'd be wasting time and money if you only have one book out.

Also, not to be a downer,  and it's cool to have a goal of writing full-time but realize probably 1% of authors (or less) write full-time. And when I say full-time, I mean making the kind of money where you can support yourself or your family or at least your writing income contributes significantly to the household. Writing full-time (even for trade published authors) is rare. Most authors will have a 9-5 (if they are lucky to have that) their entire careers.

Unless someone becomes the next Stephen King, there is no guarantee of anything. In indie publishing, writing full-time is fleeting. You can make it to full-time and then have to go back to work because suddenly your sales are in the toilet with no rhyme or reason. That's how the book world is. You can have a huge bestseller one year and then go full-time thinking it's going to always be that way, then next book you release can flop like a stone and you gotta go find a job again. That's the real world of writing folks don't talk about. Don't listen to the self-publishing cheerleaders who tell you anyone can write full-time. If that were the case, everyone would be doing it.

So while that's a great goal to have, always be realistic and take it one day at a time. Make short term goals and try to accomplish those and don't worry about the other stuff.

And, congrats on five units! Might sound like a small amount but you should be proud of that at this point. Some authors aren't selling even that.

You don't have to do anything to set yourself up in business as a sole trader. It's simply a requirement to register for self assessment if your earnings in a tax year exceed £1,000. Which means you need to keep records and accounts if you think you will get anywhere near. The £1,000 is across all self employed activities. And, once you start, HMRC will bug you every year.

Simple answer is just keep records (income is easy, costs less so), making sure you know the dates of the tax year and don't fuss until you see you need to.

If you want to separate it from your other activities, you need to set it up as a company. And there's a fair bit of cost and aggravation to go with that. Unlikely to be worth it, unless your earnings are many times higher than the average writer achieves. Of course, if you already have a company there's nothing to stop you putting your writing in it.

Thinking about your writing as a business activity or being businesslike about your writing and publishing are other things completely.