We don't have a max delivery time, so why take risks?
Is this a serious question? Paid per drop means less time means more drops means more money means they're basically encouraging their riders to go faster and let everyone else (and the NHS) pick up the pieces when they go beyond what's safe - it's the exact same problem as some commercial motorised deliveries.
They may use the fact that you were working at the time of any incident, to say you weren't covered.
It looks like the usual three cycling TP insurances don't cover it and you'd need to get a cycle courier policy:
Brutish Cycling "Who and what is not covered?... Occupational cycling and where the bike is a tool of trade."
CUK "you are covered for commuting to work and you are also covered for cycling to and from business addresses, but not for work as a cyclist e.g. as a courier or professional racing cyclist"
LCC "You are insured while you are using your cycle for private and professional purposes including business cycling, but not if you are specifically employed as a cyclist e.g. as a cyclist-messenger, a cycling trainer or a courier cyclist."
Social media policy of large organisations usually refers to, "Comments likely to damage the companies reputation" will not be tolerated, be careful.
That may why the OP didn't explicitly mention the famous "Slaveroo" nickname and hashtag.