apparently @microsoftuk is blocking access to windows 10 iso downloads ! why ?
and only allowing you to make an ISO locally with the windows media creation tool
but groovypost claim you can still get an ISO by downloading from linux instead of windows
windows 10 insider version 1803 build 17127 ISO download now available
windows 10 insider version 1803info
you can convert a bootable ISO into a bootable USB by using rufus or win32diskimage
Microsoft Windows 7, 8 & 10 (1607) (re)install isos are now finally available DIRECTLY to download from Microsoft and let you (re)install a CLEAN version of Windows 7, 8 & 10 if you still a have the licence key (often a sticker on back of your PC).