The standard UK platform height is 915mm, and a train designed for "level" boarding at one has a 960mm floor. (Yes, that sounds odd, but the step has to be slightly below the door because otherwise it would fall out as there would be nothing to hold it in place - in reality the result of that 55mm gap is two 22.5mm steps, which are achievable for a wheelchair user).
No, HS2 isn't compliant - which is stupid beyond redemption.
What we definitely *shouldn't* be doing is building trains with a higher floor than the ones they replace, which the CAF Civity is guilty of - it's very visible how many even able bodied people struggle with the massive step up to these compared to say 15x. Though it can be mitigated for able bodied people by having a separate step as e.g. Pendolinos and Voyagers do.
I must admit I have had some personal experience of this, but fortunately it is mainly good.
Recent journey from Market Rasen - Cleethorpes,
Boarding at Market Rasen onto a 170 - The passenger is fortunate enough to be able to get out the wheelchair and was able to board from the elevated hump. The guard did offer to get the ramp out.
Changing at Habrough - again 170 to 170, the passenger go on and off the train independently. The change at Habrough rather then Grimsby was because of the lift out of service at Grimsby.
Return journey was on a class 185, it happened to be stood next to a 170. It surprised me how different the stop height was. The ramp was needed to get on / off this train.
The guard on the 170 from Grimsby - Market Rasen wasn't sure which door the hump was at. As it happened we were at a door on the normal platform height which was a large step (as bad as the 185 at Grimsby). But stepping down is easier for this passenger so again not a problem. The guard offered to get the ramp out if needed.
The staff interacted with were brilliant, it was unfortunate that we were in the wrong place for the hump on the way back, but will know for next time. On thing that was a concern on the 170s is the automatic door shutting. If the guard uses their panel they disable it, otherwise someone needs to put their foot in the door to stop it attempting to close. Not sure if other modern units have the same problem.
I have found travelling with a wheelchair bound passenger my experiences of getting on / off the train are fine. The stations are more challenging with lifts out. This can be a big problem if trains skip stops. In a wheelchair it isn't that easy just to get off.