The risks of an employer being out of compliance, in any country, are that at any point in time they could be visited/audited by the relevant immigration agency. If they are found lacking in their compliance of foreign workers: e.g. record keeping, basic knowledge of the status of their foreign workers, etc., they could be fined and the applicable company license/ability to sponsor foreign workers could be revoked. In the UK, the Home Office is the responsible immigration agency that would audit an employer.