From the Tilmanstone Salads website:
"All workers are recruited through two main temporary work agencies, both of which are licensed by the Gangmaster Licensing Authority (GLA). The seasonal nature of Tilmanstone's business necessitates the need for fluctuating numbers of agency workers on a daily basis. The majority of agency workers are migrant workers from Eastern European countries such as Latvia, Lithuania and Poland."
www.ethicaltrade.org/in-action/tilmanstone-salads
It's a policy to recruit agency workers from Eastern Europe, full stop!