You would be allowed 3 schoolies. So if you were full (takes a long time to get there) you would get aprox £150 per week (assuming each stayed til 6 term time). Or £470 per week during holidys if all 3 are with you all week. But that lovely figure will be rare as parents get holidays and will look after the children when they can to save themselves money.
You can also be crafty and have children over 8 years which dont count on your numbers. But OFSTED limit you to 6 for a reason. It is incredibly hard work having 7 and you cant meet all the children's needs.
Childminding is more than just making sure they dont leave the house.
You can boost this potential with overnight care if you have the space and then you can get at least £40 per night (largest I've seen is £70 pernight). So for 4 days thats £160.
The thing to bear in mind is that costs could reduce the £150 per week to as little as £75 in your pocket. Now that works out at £5 per hour that the children are there. BUT there will be lots of other hours that you work behind the scenes such as courses, travel to courses, travel to the school to do the pickups, time spent sterilizing toys and your house, extra time tidying up, time shopping for resources, going to toy libraries, meetings with parents for reviews, doing paperwork for accounts and education, reviewing policies, reading revelent books/publications, laying out activities, extra washing up......I could go on and on....You'd have to be desperate to do it for a little extra money but you might do it to keep busy and gain skills and the enjoyment of children. In fact if you dont enjoy children (I have met some cm that for all appearances hate children - 2 so far) then you should forget it anyway.
Sorry its so long but no one else seemed to have answered the questions.