Set Win10 Reg Key For Ethernet Metered Can't Get Permission

Set Reg Key for Ethernet metered needs permission.
Windows 10, RegEdit, Permissions, Administrator, Current User
I wanted to change a Reg Key called DefaultMediaCost so that all internet connections were set as metered to stop windows from downloading updates.
After trying various solutions I always came upon the "unable to save permission changes" message.
Eventually I found a post suggesting that instead of trying to set permission to Administrator or Hidden Administrator I should set to Current User found using Command Prompt>Net User.
On my computer RegEdit found L850\Administrator (which did not work) but after doing the net user command above I tried L850\Toshiba which it seems is the current user.
The 3 users on the machine are Administrator, Toshiba and Guest.
L850\Toshiba worked and I was able to set the Ethernet Key to 2 instead of 1.
The reason for this is I plug in a Mobile Broadband Dongle via USB which comes up as a Ethernet connection. I could plug it into power separately and use the WiFi but then I have to mess with extra cables.
When I get home I may not be able to update windows with my optical fibre internet so will have to set the key back again to 1?
My question: is there an easier way of stopping windows chewing up all my expensive Mobile Broadband GBytes?
Note: I also tried SetACL.exe and Subinacl.exe and a reset.bat file but windows prevented these from working in system32 as suggested from Command prompt (admin).

 

Have a look over here.
http://www.askvg.com/tip-force-windows-10-to-use-ethernet-connection-as-metered/

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[Guide] How to Take Ownership (Permission) of a Registry Key in Windows?

[Windows Tip] Take Ownership (Permission) of Registry Keys from Command Line

 

Disable the Ethernet e.g. on a laptop.
Plug in the 4G device via USB for power but now ethernet does not work so connect to it by Wifi.

 

Thankyou Eldon for the link to registry key ownership information.
My Toshiba laptop updated OK when connected to home LAN