Employers are responsible for ensuring that they are nor employing workers who do not have the right to work in the UK. The employer is required to undertake (and show compliance) checks on workers to establish their entitlement to work before they commence employment.
The consequences can be grave for employers who fail to comply. In one recent case rather like lightning these consequences struck in more than one way. A restaurant owner employed two illegal workers as they were not eligible to work in the UK. The Home Office Immigration Enforcement Officers visited the restaurant so they were caught in the act so to speak. The business was fined £20,000. The company then went into liquidation. Finally the director was then subject to a disqualification for employing illegal workers and banned from being a director of a company or being involved in the promotion, formation or management of a company for six years.