I'd say £35-£40 per hour is ample. I personally would feel it was morally wrong to go higher. If you tutor at £35 per hour three times a week, you make £105 per week, or £420 per month. If you are doing that on top of a salary (as I am) then it is more than reasonable as a supplement to your income.
Even if you tutored for 20 hours a week (i.e. effectively doing that as a full-time job; not saying more than 20 hours as you may need to account for travel time), you'd make £700 a week, or £2800 a month before tax. That's not doing badly. You can always make more than that if you want (I know some people who work all the hours God sends...not that I think you should, as it becomes bad for your health and wellbeing), but that's really not a bad beginning.