It is the CAA that imposes that as a local requirement.
My understanding also.
The safety argument for microlights not having to use a licensed aerodrome is because the aircraft mass is below 450 Kgs (originally 390) and the quantity of fuel they can carry is restricted.
Err, run that past me again. Fuel state isn't restricted now (and previously was limited to 50 litres, which wouldn't take much effort to create a decent fireball anyhow). I believe that microlight schools are bound to rules about runway length, firefighting equipment, etc. anyway - what would a license add to the safety of a system that requires an FI to ensure that the airfield is suitable anyhow (as I'm sure any FI would, regardless of rules).
Out of interest, apart from 2 letters, is the difference between a TMG and SLMG - they both sound like Grob 109s to me?
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