From my experience of the tax man, running a flying school, and paying instructors, i was told by the Inland Rev, that Instructors can only be employed by the company. They cannot be self employed unless they provide their own aircraft and do not take bookings through a school/club. I know this isn't always the case. My advice would be to contact IR.
Most schools pay by the hour typically 15-20squid per flying hour. Some pay you for groundschool, and some will give you a daily retainer. Every school is different. You can get upto 17k a year, but will have to work extremely hard to get it!
All the very best to you as an instructor, I love it!
JP