My question is, when identifying yourself to the ATSU, you state that you are IFR, 2800ft requesting RAS, giving all other relevant details. Does this constitue an IFR flight plan as filed over RT, thereby enabling the ATSU to issue you with a radar service (RAS)?
No
Definition of a Flight Plan - "Details of a Flight or Portion of a Flight to enable the issuance of an ATC clearance".
IFR in Class 'G' does not require a clearance, so does not require a FPL, so this isn't technically a FPL. If you were crossing CAS at all and that was included in your request then it would
I am trying to clarify the rules in my head here. As I understand it, if you are IFR, the only time you need to pre-file a flight plan is if you will be IFR in controlled airspace (not class A if you have an IMC rating only) or can this also be passed over RT?
Yes it can - see answer above
When cancelling IFR in uncontrolled, I also believe that the words "Request cancel IFR" have to be initiated by the pilot before the ATSU can downgrade to an FIS or RIS?
No - they're not. Again, no clearance required, so you don't have to "request" anything. You state you are changing back to VFR and downgrade the service accordingly.