But do you do it as a business, in business premises paying rent and business rates, and VAT, and National Insurance and income tax and public liability insurance and so on?
Lets assume you ran a bike shop. I'd imagine most of your income would come from repairs rather than sales because you wouldn't be able to price match the bigger companies. And running a bike shop is stressful and high risk, so you'd want to earn a decent amount. So let's say you want to earn £25k a year from the repairs side of the business, and you spend 30hours per week doing the repairs (and the rest of the time doing the books, serving customers, answering the phone, selling bikes, ordering stock etc)
That means you need to earn £16.66 per hour doing repairs just to earn your £25k. You need to add to that the cost of your rent, rates, utilities, insurance, VAT etc.
Which all of a sudden makes £25 to supply and fit a gear cable inner (and, I suspect, clean out the outers) seem not so bad to me.