Affiliate Marketing Secrets

I am a full time employee. As a hobby I’ve started creating web pages on free platforms (free hosting and free sub-domain); those pages also include affiliate links from Amazon or eBay. Let’s say I will, at some point, make £20 per year. Do I have to declare myself self employed, start filling in self assessment forms and pay tax to Inland Revenue for £20 of additional earnings (on top of my annual salary)?
If not, being a negligible amount, then what is the threshold (if any) above which you have to declare yourself self-employed and start paying taxes? If there is no exact threshold, does this mean that the Inland Revenue is expecting getting taxes from (let’s say) a person that makes online a £1 profit (paid into his bank account) from selling one item on eBay?
Thank you.