If you look back in this forum there was thread on pay recently.
To summarise though:
Pay between 10-20 pounds per flight hour. Basically 10 grand per year on average.
You will probably be self employed and therefore the number of hours worked/holidays taken will be linked to how busy the school is/you want to be.
I really like the school I work at, but you don't have to read this forum for too long find stories of poor/terrible conditions.
Instructing can be really good fun and I believe it is the most realistic way of improving your chances of progresssing in aviation (I bloody hope so anyway).
FIS