At my previous school in Hertfordshire the flying instructors were paid £15 per day retainer and also £15 per flying hour, if you taught ground school for an hour that was also paid at £15 per hour. Other instructors who were not on a retainer were paid £20 per flying hour.
I completed 700 hours in 11 months and earned just over £15k this is pre tax, you will find most instructors are self employed and must sort out tax, NI etc themsleves.
Hope that helps.