“Asian” in UK, not unreasonably, is generally used to refer to the largest group of immigrants from the continent of Asia, who are from South Asia: Pakistan, Bangladesh and India.
The same term in USA, equally reasonably, is used to refer to our largest group of immigrants from that continent, East Asians: Chinese, Japanese, Vietnamese, etc.
The term just means different things in the two countries.
Ah, so! That makes sense.