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.

Why did Wells write the story that way? Well, because he was writing for an English audience, that's why. No more to it than that.

But I do find it amusing how much the English thought the world rotated around England at that time in history (the French no doubt thought the same about France)....and so, of course, if the Martians were intending to conquer the Earth they, with quite limited numbers of their machines deployed, would have to start by taking over the British Isles, obviously. It was, after all, the center of Earthly power at the time, and it would also make an excellent initial base of operations, easily defensible once taken over, and then they could branch out from there to take over the rest of the planet, section by section.

Were the Martians Anglophiles? Most certainly! Who could doubt it? They considered good old Anglo-Saxon blood infinitely preferable to other, lesser varieties of Earthly essence, and this was clearly evidenced by their cunning assault upon the British Isles at the very height of British Imperial power.

And a jolly good time was had by all!