Most British startups start out building products for the British market. So they have a 10-20 million target. Once they achieve major success, they figure they'll go overseas.
U.S. startups start out building products for the American market, so they have a 300 million target. And since our version of English is much more prevalent online that target is even higher.
Since growth rates are more or less the same, you have British companies at a disadvantage. While a US company might have 10 million users after 5 years, the same exact company in Britain would only have 100K users.
The market share may be the same, but due to the sheer size of the market, the U.S. company has a lot more customers. And once those customers start paying, the U.S. company has a ton more $$$ to play with to grow even further.
There are obviously a number of exceptions, British companies doing well in the American market. But those are the culturally aware companies, that realize that the British culture is different from the American one.