Question to the Home Office:

To ask the Secretary of State for the Home Department, what data her Department collects on the nationality of people spot-checked by immigration officers and subsequently (a) arrested and (b) not arrested.

The nationality of every individual arrested during immigration enforcement activity is recorded as part of their immigration records.

https://www.gov.uk/government/publications/powers-and-operational-procedure

The nationality of individuals spoken to or encountered but not arrested during deployments in public areas may or may not be recorded.