We are not building new platforms on the HS2 line at a lower height than 915mm. Rather, at the new HS2 stations, platforms will be circa 1115mm above rail height. This is higher than most current UK Network Rail platforms (which are nominally 915mm above rail height, though, for historical reasons, this can vary significantly).
The higher platform height for HS2 was chosen to improve passenger accessibility by achieving step-free access between all new HS2 platforms and the new high speed trains. HS2 Ltd, in cooperation with the Department, consulted international rolling stock manufacturers on the design and technical requirements needed to provide step-free access for HS2 and simultaneously achieve HS2’s target capacity and journey times. This consultation indicated that specification of an increased platform height was necessary.