Earlier quoted context omitted.
Ha. I'd be interested to see anyone manage that. On top of this, in central London it's pretty built up so SatNav doesn't always work in the canyons between buildings.
Plus a SatNav won't know that at rush hour, a longer (distance-wise) route could be quicker in a black cab due to them being allowed to use bus lanes.
Also, even if GPS signals are hard to get in London, navigation in a fixed city can be calculated in lots of different ways. Allowing for some regulatory limits, a company (or an experienced third-party) could scatter their own transmitters around the city as needed, figuring out which frequencies give the best accuracy.