'Upgrading' to skinny tyres pointless for London commuting

You need to factor in the weight, 80kg in says a 5mm square contact point vs. 80kg in a 1cm square - which would you reckon spread the weight evenly?

Pressure = force per unit area

Bigger tyre = bigger contact patch

Common misconception. In fact the pressure between the road and the tyre is pretty much exactly the same as the pressure you inflate the tyre to whatever tyre size. If you put more weight on the tyre then the contact area spreads out to keep the same pressure.

So, for a larger and smaller width tyre at the same pressure with the same rider they will have the same contact area. The key to rolling resistance is the shape of the contact area (I suspect Tester has already covered this but here it is anyway). The thinner tyre has a long thin contact area whereas the wider tyre has a wider shorter contact area.

Rolling resistance comes from the losses caused by internal friction in the tyre and between the tyre and inner tube so the more you bend the tyre, the more energy you lose. If you imagine the contact area as a flattening of the circular shape of the tyre against the road then you can imagine that it's much harder to flatten a long thin strip than a short fat strip because of the curvature of the tyre. So the long thin strip causes larger energy losses in the tyre (at the same pressure). For the same reason, larger diameter wheels have lower rolling resistance due to the increased curvature of the tyres and higher pressure reduces the length of the contact strip and will also reduce rolling resistance (on a hard surface)

But it's all a play off between aero, weight, rolling losses AND comfort so there's no one size fits all answer.