UPDATE May 6th, after the release of the ADMX file for Windows 10 1803, the GPO settings to manage these restrictions are available
With the Windows 10 April Update (build 1803) you finally can manage the amount of bandwidth being used by Windows Update.
To manage the limits (applicable to both download and upload [aka sharing with internet clients] – knowing that if you have already restrict the sharing with PC on your local network only this will not be applied), open the Settings\Windows Update\Advanced Options
Then scroll down to locate the Delivery Optimization option and scroll down again to get the Advanced Options
From there you can define the bandwidth limits for both download (separated into 2 section – background and foreground download) and upload.
The limits also include for the upload, an amount of maximum data shared.
The minimum you can set is:
- 5% for bandwidth usage
- 5G for data sharing with internet client
To define the restrictions by GPO you need to have the Windows 10 1803 ADMX files (https://t.co/93MJI3a1rX). The GPO settings are located in the Computer Configuration\Administrative Templates\Windows Components\Delivery Optimization
It does not seems there is GPO setting available (as looking at the local policy as ADMX files for Windows 10 1803 are not yet released)
UPDATE thanks to my colleague Karl Gilbert who took a look as I did not 
, here is the registry keys associated with theses settings. You can then push it with GPO or script
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\DeliveryOptimization\Settings
DownloadRateBackgroundPctDownloadRateForegroundPctUploadLimitGBMonthUpRatePctBandwidth