What should aspiring writers know about self-publishing books in the UK?

I’m going to assume you actually mean self-publishing and not the vanity press. The latter is expensive, not well regarded in the trade, and I personally wouldn’t use it.

I am also going to assume that you have tried getting a literary agent or submitting your work directly to publishers and have enjoyed no success. This will certainly happen if your writing is not good, but it can also happen if it is not commercial and even if it is good but can’t get through the less than perfect filtration systems that publishers employ to shield their top people from the great mass of submissions that flood in. Remember, these days half the world’s population believe they can write.

The explosion of social media and internet facilities in the last couple of decades means, provided you are willing to do the formatting required by the particular form of publication you choose, that it is not a complicated matter to publish an e-book and only slightly more complicated to turn that e-book into a print-on-demand paperback.

There is no point in my setting out a detailed list of instructions since online firms such as Amazon and Smashwords already provide such guides, and you can also download free software to help you with the formatting.

It does help if you can get some impartial beta readers to look over your work before you do this. For the purposes of finding straightforward errors, wrong punctuation, and so on, it doesn’t matter too much if these are family and friends who will do the job for nothing. For the purpose of telling you that your work needs editing, you will probably find things are different. There are writing clubs that may help for nothing; there are of course professional editors whom you will have to pay. I personally got a lot of good advice from membership of an online club where members in good standing exchanged critiques with each other.

But getting your work self-published is the beginning of the process, not the end. It may be out there, but it is invisible. The half of the world that think they can write are self-publishing alongside the relatively few people who actually can, and your book is not likely to stand out in the crowd unless you make it.

Nobody is going to market it for you. If you don’t do your own marketing, it will be unlikely your book will sell in significant numbers. Marketing is considerably harder work than self-publishing, but that’s not what you asked about.