In my area, Childminders charge around £5 per hour per child for after-school care. I'm in West Surrey.
If you provide the childcare at your home, and your friends children are aged under 8, then you need to register as a childminder. In Surrey, contact the Childminding Development Officer on 01372 833826 for an initial discussion. Your registration would be for a certain number of childcare places... you don't have to fill them all.
The way around the issue, is to care for the children at their own home... then you are a nanny. Babysitting is the same as being a nanny - at least, I can't find anywhere official which states otherwise.
Childcare provided in your home, between 6pm and 2am is unregulated in England - it is only care between 2am and 6pm which is regulated (by Ofsted in England) and applicable when care is provided for 2 or more hours in any day (day in this situation, being 2am to 6pm). Appropriate legislation for this is mostly to be found in the Care Standards Act 2000.
When doing favours for friends, it is the 'reward' aspect which is important. Anything may be considered to be reward, not just money - so a box of chocolates, bunch of flowers etc. One thing I would expect the regulator to take into account, was the frequency that care was provided. The same time each week day, would not in my view be friends coming over to play. Friends coming over to play for a couple of hours every now and then, is another matter. There is provision within legislation for there to be 5 occasions in a year, in which care is provided of 2 hours or longer. It is when such care is provided on more than 5 occasions, that it becomes a breach of the regulations.