Actually, it is quite clear in the story that the landings are confined to only 10 locations in England. First of all, the astronomer, Ogilvie, observes a series of 10 very large explosions happening on the planet Mars over a 10 day period. None after that. Those are the launches of 10 spacecraft by the Martians, and these arrive sometime later...I don't recall how many days later or even if it is said, but they arrive in an orderly sequence, all landing in England.

An interesting, if inconclusive, reading.
English astronomers would only observe rockets launched during their time of viewing the skies (English night time). It is quite possible that many more were launched over the ten day period, but the number seen being launched coincided with the number reported as landing in England.
This would also explain why Orson Welles account sees a similar number launched from Mars and landing in USA. :)