Maximum permitted Insulation Resistance values HV

On what? a transformer? A single insulator., dry prior to installation?

an overhead line of several km, exposed to wind and rain, same again in a coastal location? All 3 will be wildly different .

A transformer bare and brand new should be some few hundreds of megohms but core grounding methods and internal components such as switches, fuses, and taps may lower  the results of insulation resistance tests.

A bare bush or  insulator will be gigohms dry and brand new. Covered in salt spray and struggling, a lot less !! 

 Brand new paper or XPLE Cables are normally charged to a DC comparable to the peak AC and the  leakage current monitored for some minutes. There will be some dielectric absorption, but current should settle to well less than 1mA per km. Once in service, it can get a lot worse before things need to be replaced.

DNOs usually have their in-house standards  example https://www.spenergynetworks.co.uk/userfiles/file/SUB-02-613.pdf

Mike.