You can't really cross oceans with microwaves though. So a London from can gain a competitive advantage against other London firms on Wall Street exchange.
Cables run relatively straight under the sea. If they are optic fibre, they run at a lower speed than light in vacuum.
Satellites are in space, so communicate pretty much at speed of light, but probably not in a straight line, depends where the satellites are.
HFTs are indeed great at gaming such things, will be interesting to see what happens.