My AW hub is ideal for south Birmingham with its 80, 60 and 45 inch gears. London is flatter so I reckon you should be OK with a 5/6 speeder. I'd use the chainring that give the best chainline.
Whilst most of London is fairly forgiving, gradient-wise, it nonetheless does have a number of locations where having to get off and walk up is a real possibility if your bottom gear is relatively high. It's a fallacy that London is flat, just in the same way Norfolk isn't flat either.
When you look at the ratios on a lot of bikes with a high gear count, what you find is ratio duplication, or ratios that are so close as to be pointless - such as two gears that are only an inch or two different. Personally I don't see the need for more than 10 well-chosen ratios, What I think has happened, especially at the low budget end, is that manufacturers have engaged in a ratio "arms race" as a selling point. They use the fact that their bikes have 18 speeds as a marketing tool, despite the fact most buyers of such bikes won't ever use half the gears offered! Rather than make a big deal about selling £100 cheapo bikes with 18 gears, they would be better off selling £100 bikes with just 5 gears and using the money saved from the multi-chainrings and front changers to improve the component quality.
Looking at the rust/wear on the chainrings of my salvage 18 speeder, it's clear that the smallest chainring has never been used at all, the middle one is little-used, and the vast majority of it's miles have been clocked up in High range.