Don’t put a timber floor in. It will sweat like mad and the timber will rot unless you ventilate the floor. Do as Nick says - floor finish on VCL on insulation on DPM.
Waste of money insulating the garage if it’s not heated. Insulate the wall between the garage and utility and the floor between the garage and any habitable (and heated) accommodation above. No heat = no insulation required.
If the radon maps says there’s radon in the area put a radon barrier down. What benefit is spending a shed load of money on an “assessment” which will more than likely be a desktop exercise using the same maps!
Rubbish. It’s the cost of a 1200 gauge DPM with all joints sealed and a few top hats. Put it under the slab and you don’t need any radon proof DPCs at all.