Just for clarity, a BB cannot be used in Westminster as regards the waiting exemption but would provide strong support for the need for assisted alighting.
IMO, the sign is compliant because 'Midday-7am' is a permissible format. The three panels are placed correctly and there's no overlap between times.
But I cannot see a yellow line or kerb blips in the photos of the location, in which case the markings are improper because the restriction cannot be conveyed by signs alone.
OP, as regards your claim to the exception of assisted alighting(assuming the markings are there) the burden of proof falls to you. In this respect, you would have to show to an adjudicator - I think the council would reject- that you came, you parked, you went to the cafe, you immediately returned with the passenger who needed assistance. Do you have anything to corroborate your account e.g. dashcam record etc?