Are you looking at working for yourself or working for a gym or something like that?
You will need to do a course, whatever it may be in the UK and have your first aid up-to-date.
If you're working for a gym or some place, the money is not spectacular, unless you are somone that really drags in the clients.
If you work for yourself, you can basically charge what you like, but then again, it's up to you to find clients and fill in your day.