If one is expecting a procedural ILS then the airspace is likely to be controlled.
May or may not.
Class G ILS: Lydd, Biggin, Exeter, Newquay, Cranfield, Cambridge, Norwich, etc.
Class D ILS: Bournemouth, Southampton, Gatwick
, Birmingham, etc
Filing a written flight plan tells ATC that you are coming but it gives you nothing in the way of a more likely clearance to enter CAS.
People here normally file implicit ones i.e. a radio call to the Class D unit.
CAS makes little difference to the chance of getting it. Cranfield (Class G) more or less tells people to sod off unless booked in way in advance - as well as charging £60 for a landing+ILS
The bigger issue is airport PPR e.g. Bournemouth asks for PPR and they give you a PPR number. Why? Job creation.