As long as you are not being paid you can "baby-sit" for HE friends, iyswim. My HE friends and I regularly "swap" our kids to help out, purely as support. Bit of a grey area but it can be done as long as it's not deemed for financial gain.
I know grandparents who HE the dc as the parent has to work, for example.
Home educating a group of children could be more tricky as there are rules concerning the definition of a school. It has been described as a place where children get the major part of their education for more than 15 hours per week, with more than 5 children (not of your own family)or 1 child with special needs (I think that was the wording, but best to check it out). If your group falls within the definition of a school then you become liable to all the regulations concerning running a school, Ofsted inspections etc, so best to avoid that