Self-employed rates are just completely different.
If this was an hourly rate for a permanent employed role with contracted hours – great.
In this situation, you know when you’re working, the hours are guaranteed, you just show up and go home. The employer takes care of timetabling, PAYE, marketing, contracts, chasing payments, customer service, handling enquiries, acquiring clients, dealing with complaints or queries. All of that is a LOT of work. And it’s not billable to anyone.
If on the other hand you’re a self-employed cleaner, then filling an entire working week with long-term regular clients at possibly as little as 2 hours per booking is a whole world of work outside of the cleaning. And any number of those people could cancel the agreement at any time and you’re back to square one. You might not be managing to get enough hours in any given week. People will also mess you about a lot with things like last minute cancellations, messing about with payments, trying to change their slot, etc, etc. The cleaner will also have unpaid travel time (and costs) in between each clean, in addition to covering the cost of cleaning supplies.
You also don’t have holiday pay, sickness pay, pension contributions, redundancy, or any of the other benefits you might get in an employed role. On top of the fact that in an employed role you can’t just be fired for no reason, whereas as a sole trader there is zero job security and no guaranteed hours.
£21 hour as a self-employed service rate is incredibly low.