I am a little confused here.Heating or insulated stud walls are irrelevant to the basic best practice - The Bldg Regs require that insulation goes into all interior pours.
The Bldg Regs are for now, and all future owners of the property
are you saying, even if it’s not building regs compliant, it is best to insulate under the floor to future proof? ( although that wouldn’t help as part L compliance wouldn’t help make an exempt structure compliant)