Buster the Bear
A specific "approval" to fly a GNSS approach is not required. It is recognised by ICAO, and thereby as a Contracting State the UK accepts it as a permitted method of flying Instrument Approach Procedure. Given an alternative of an NDB or GNSS instrument approach procedure its no contest which one I'd fly.
Why are GA airfields paying to have GNSS approaches? Maybe because they are a cost effective method of providing an accurate IAP with little or no ground equipment to maintain?
All that is required, apart from SOU getting their act together and paying for the procedure design and CAA acceptance, are aircraft that are suitably equipped and crews that are suitably trained.
All, current generation of public transport aircraft are GPS enabled.