The British were the colonial rulers of southeast Asia. As a result, there may be significant genetic mixing. Also, because of GB's historic ties to southeast Asia, a lot of Asian's from that area live in the UK. Indeed, according to Wikipedia, almost 7% of the UK population is Asian.
There is a huge genetic difference between Southeast Asia and South Asia.
South Asians are Hindu/Muslims from India, Bangladesh and Pakistan. In the UK, they are known as Asians. Chinese, Malaysians, Vietnamese, Japanese, and Koreans are known as Orientals.