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I've ridden around London and Edinburgh a fair bit. A 3-speed city bike is ideal for most of London. Fairly flat and due to congestion you'll not get much benefit from any type of road or off-road racing style bike.

Edinburgh is quite hilly and less congested. I still prefer an upright city bike but with an 8-speed or N360 internal geared hub. Given the hills a hybrid does make a bit more sense here though the drawbacks don't outweigh the advantages for me.