Apart from the fact even standard platform foul the loading gauge for some locos, even after taking an inch off the buffer beam. A class 69 has run through the 1,115mm platforms of the core. Plus only a few wagon types would need to be approved passed Old Oak Common, as it is only stuff to Paddington New Yard and engineering trains to worry about.
Class 69 in Core
You posted that before, and I believe you that the cl69 fits. That one loco fits does not mean all other locos do, and the 69 is a rather niche type at that. More importantly, the the freight wagons, and everything else also needs to fit. The only way to do that is to respect the loading gauge.
You're saying the standard British platforms foul the standard British loading gauge? That sounds rather unlikely!
Paddington High level platforms 6 & 7 have level boarding for the 387s. Network Rail are currently rebuilding Ealing Broadway to reduce number of falls on to the 750mm platforms. A lot of the GWR platforms are 750mm, to not foul the loading gauge. Also 915mm standard fouls the European gauge found on HS1 & HS2.
Are you sure about the 750 mmm because that is very low, even below the EU standard of 760mm? Maybe some units got mixed up?
HS1 of course has separate "European" platforms and UK platforms, which is a very sensible solution on that railway - it enables trains that fit the standard uk platform height, that continue onto conventional UK railway lines, while also enabling EU-standard international stock for international services. HS1 is a special case because of its link to Europe, no other railway in the UK is like it, or ever will be like it.
I'm trying to think of why you're bringing HS1 up, because it is so clearly irrelevant. Unless you are proposing fully separating the Lizzie line and HEx from GWR? But if so, then why not say that?