Leide nicht.
Trains in Germany and the UK for main-line running both use 1435mm gauge. UK trains are not a custom size.
Everything about UK rail is custom (apart from the gauge). Apparently it's one of the (many) reasons HS2 is such a mess.
They were trying to run trains faster than typical continental high speed lines, which meant custom design work that needs loads of additional testing and certification. Rather than just use the Spanish or French high speed designs.
https://en.wikipedia.org/wiki/Structure_gauge / Lichtraumprofil are different.
They are. Rail gauge and loading gauge are different concepts. Use Wikipedia.