Stepping distance requirements are as follows: (From Railway Safety and Standards Board Click
HERE) 450mm would be nearly double the maximum allowance!
With so many different rail vehicles out there, it's very hit and miss with achieving the 230mm step height whilst ensuring all vehicles (non passenger too) are clear of the platform edge. At Tamworth on P2, if I remember rightly, the track is on cant transition with the cant handing away from the platform edge and thus the footstep tilts up in the air. Was it at the Lichfield end of the platform as this is where the maximum cant is before rounding the bend into a straight towards London? Could be a number of things. Bad design (Would be difficult to get such a stepping height through any gauging engineer!) Possibly the track that has been tamped too high but, again, would have to be a major muck up to over tamp it by the amount you say.
The RSSB standard I've linked to is principally for rail vehicle designers to comply with. New vehicles should comply with the 230mm vertical step height to a standard platform height of 915mm.
There is more focus on achieving a standardised platform heights and offsets (rail to platform dimensions) rather than optimising the stepping distances for the vehicles which stop at the station in question. Going forward, it should result in a more consistent stepping distance with new rolling stock, which up until now has been a bit woolly.
Have you checked your knees haven't changed in height?