Of course there already is a UK one - accepted in many, many countries worldwide.
Its the NPPL (microlights) Instructor Rating.
My guys get paid 35 quid an hour and have no option but to make FI a career!
The old microlight medical was basically adapted as the NPPL medical.
Why not now adopt the microlight FI standards?
ps. As another thread shows, it is possible to be an FI microlight, train someone up to test standard and then - using a CRI hat - give them three free hours training in a light aircraft version of a microlight (ie Group A C42 Ikarus or Eurostar) to get a NPPL SSEA. So the route is almost there - you just can't get paid for the last three hours (but three free hours flying after you've paid for 40-odd isn't too much of a hardship!)