I need to get macOS High Sierra dmg file to restore my 2011 mac which has no operating system

You can boot into Internet Recovery Mode using Command + Option + R to access the online macOS 10.13 installer.

Attempting from a Windows computer will not work, shall not and will never work.

The only way is by using an Apple Computer that qualifies to run High Sierra.

If you can access some other Apple Computer that qualifies to run High Sierra - that might be the solution even if on a temporary bases to create the Bootable Installer

Then when that s done >> see below instructions

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as HFS Journaled with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

You can get the download you need from the App Store.

You should only use Safari to download this because Firefox and other browsers may not work.

macOS 10.13.6 High Sierra at the Mac App Store - Apple:

click> macOS High Sierra 10.13

Also best to use an ethernet connection to your router for this rather than rely on wi-fi.

I am in the UK and of course it is still available,

you need a mac to download it.

Can you borrow one.