Wow I should tutor! I've left teaching and was looking at minimum wage work!
A lot of people also say that about gardening, cleaning, etc., but soon realise the reality. The difference is that with "a job" you get paid for all the time you spend. Whereas with self employment, you have a lot of unpaid time, such as administration, driving to customers' homes, time spent on advertising/marketing, meetings with the accountant. Then of course the extra costs, such as insurance, travelling, accountancy fees, stationery, computer, books, subscriptions, etc.
For my job, as a self employed accountant, I charge for about half the time I actually work, so my "charge out rate" is automatically halved. Then out of that, my overheads eat up another half, so my real "wage" is a quarter of my charge out rate. This is typical of many self employments - I have a spreadsheet which I go through with new clients to pop in the unchargeable time and overheads, and it usually ends up between a quarter and a half of the charge out rate being the actual hourly rate!
So, given minimum wage at £7.50, a charge out rate of £30 per hour isn't going to be too far from the mark as an equivalent to cover the non chargeable time you'll have to work and all associated costs, such as travel, phone, professional fees, insurance, computing, stationery, books/subscriptions, etc.