Well it was £40 a few years ago. And speaking to another ex instructor the other day he was on £35 whilst restricted, and £45 when unrestricted.
However, from time to time there seems to be these abnormalities of people on way below what they should be simply because they work for one school and don't have contact with many others in the same line of work.
£50 sounds like a fair-to-high rate for a freelancer with the current cost of licensing and availability of instructors.
The rule of thumb always used to be:
£40 instructing
£250 single
£300 twin
£350 twin IR