If you can afford Hi-octane wihout feeling a pinch you should get that , otherwise you can do 50/50 or 70/30 blend for better running . Honestly I'm using just normal for a few months now since fuel prices just went insane.
1000 km inspection isn't v important they just lift the car up check nuts bolts etc , If I was you i'd check the date when my car was delivered and see how much time period has passed , if its has been more than 6 months you should definitely get its engine oil and filter changed.
For service go to any MG 3's and don't forget to get your warranty book stamped after inspection.