The U.K, has a workforce of 35 million. About 2.9 million of them, or over 8%, have migrated from the E.U. before Brexit and are eligible for shifting to formal post – Brexit immigration status.
This is equivalent to half of the foreign-born workers in the U.S. having to adjust their immigration status in the space of a few years, and with a severe deadline for applying for adjustment.
The penetration of the EU workers is very high in some sectors. “In 2018, according to the Migration Observatory, 21% of low-skilled factory and construction jobs were held by EU-born workers, as were 17% of factory and machine operators jobs, and 13% of jobs in food preparation and ‘other skilled trades’. People from older EU member states tend to be more concentrated in the education, and professional and scientific sectors while workers from newer member states tend to be more concentrated in the retail, and transport and storage sectors. In 2018, more than half of workers from newer EU member states were overqualified for the work they did, compared to less than a quarter from older member states.” (from here.)
Also data from here and here.