No doubt £££ has a lot to do with it, as well as a lack of availability of service aircraft (Back in the day they always borrowed a 115 Sqn Andover, but they have used a civillian Skyvan, C130 or Chinook for many years).
Their 2017 brochure mentions this, although they need to explain to their graphic artist the difference in Chinook length with and without rotors!
https://www.raf.mod.uk/falcons/rafcm...102BB96E82.pdf
The 228 has been used for civillian parachute operations for many years