While the installation is running (or better, let it fully complete), navigate to:
C:\Program Files\Epic Games\UE_4.27\
Also check the temporary download cache folder (usually hidden):
C:\ProgramData\Epic\EpicOnlineServices\DownloadCache\
C:\Users\[YourName]\AppData\Local\EpicGamesLauncher\Saved\Downloads\
However, this is messy. The cleanest method is to let the launcher finish installing, then copy the entire installed folder to an external drive. This is a "portable" offline version.
Solution: UE 4.27 requires Visual Studio 2019 v16.9.8 or later. Download the Visual Studio Community 2019 Offline Installer (vs_community.exe --layout) separately.
A community-led project called UE4 Offline Installer Archive tracks every release back to 4.7. For 4.27: