A large truck with 2 staff would have done the job faster. I use to live next to Lambeth, there is hardly any traffic that will impede a large truck doing distribution.
Again, I disagree.
It was an operation to drop off care packages, presumably meaning one or two boxes to each household. Lambeth has modal filters as part of the Low Traffic Neighbourhood they have implemented. One lumbering truck driving around residential streets, navigating parked cars and tight corners would have taken a lot longer I think. Not only can the cargo bikes get through modal filters, they're easier to park and they'd have multiple people doing deliveries in parallel for a lower cost than running a single truck. I really do not think a truck and two staff could deliver 400 care packages a day in a built up area. When I've received deliveries from parcel companies I think the highest stop number I've been on a route is something like 100 or so.
I think 400 parcels a day for two people and a truck in a residential area is extremely optimistic. If they worked a 10 hour day that would be a delivery every 90 seconds. Even a 14 hour day would be a delivery every two minutes and six seconds, and both those calculations exclude breaks, lunch, and so on. Based on those calculations I think you'd need at least 4 or 5 separate vehicles to do that, at which point you'd be at ten staff in pairs doing a delivery every ten minutes roughly, with hopefully enough time for a break.
Given the running costs of 4/5 vans/trucks and ten staff, I think it's pretty reasonable that a set of cargo bike riders could do it cheaper and more efficiently.
EDIT: Clearly e-cargo bikes aren't as effective a solution for huge sprawling cities full of stroads, multi-lane highways and a wasteland of empty car parks, or indeed rural areas, but I strongly believe for relatively compact built up cities and residential zones like what London has, they're the future.