Generally platforms can only be raised to be level with a typical train entrance (about 1100mm above top of rail) if the track in question isn't used by freight trains, as they will foul the higher platform. So it's only seen in a few places - Elizabeth Line central section, East London Line, Heathrow and the Paddington HEX platforms, and many stations on the Underground sub-surface lines (and possibly a few others I don't recall). The other option is a train with lower entrances, but Thameslink doesn't have these.