It's a lagging indicator. A lot of those South Asians tend to be 2nd or 3rd generation whose heritage is from the Doab and Mirpur who came to work in factories like the Michelin tyre factory in Southall, the Ford factory in Dagenham, and the textile mills up in Birmingham and North Yorkshire in the 1950s-70s. South Asian immigration to the UK slowed down by the 90s when America and Canada became easier for South Asians to emigrate to.