I used to be a SE cleaner for a number of years.
You must register as self employed with the HMRC for self assessment purposes. Even if you're only going to be cleaning for 2 hours a week and earning £20, you must still register within 3 months of 'starting your business' as they call it.
I charged £8 per hour (Glasgow-ish area) and supplied my own products. I also don't drive, so walked or used public transport. I had all my cleaning stuff in a little 'granny' shopping trolley which had my business name and contact details sewn onto it.
i also advertised locally and had some leaflets printed out by vistaprint. I also got two t-shirts printed out to match the leaflets, which i wore to my cleaning jobs, visiting potential customers for quotes, and delivering leaflets.
I charged by hour, not by tasks/roomsize. They just listed what they wanted me to do for the day and how long they wanted me for, and i cracked on. If, for example, they wanted me to clean the inside of their oven and microwave, depending on the state of them, this might be all I'd manage to do in 2 hours.
I took out insurance, but never had to claim on it thankfully. It cost me about £70 for the year and i took it out mainly to put my customers at ease.
You must think about how you will be paid. Cash? Paypal? Bank transfer? Cheque? And you must keep tabs of all incoming and outgoings for HMRC purposes.
You should be able to manage your 'books' by yourself unless you get ludicrously busy and have to emply staff etc. In that case, you may want to look into an accountant.
I'm happy to pass on any advice if you want to know anything?