Yes, you are getting it right. IRIs are going to be a very rare commodity in the future.
The Instrument Flight Rules outside controlled airspace do not include the filing of a flight plan whereas inside controlled airspace they do.
With certain limited exceptions, you must fly above MSA and, if above 3000ft, in accordance with the quadrantal or semicircular rule as appropriate.
This will all change on 4 Dec next year when Part-SERA comes into force in the UK.