Earnings must be aggregated and NICs paid on the total earnings if an employee receives earnings from separate employments with a UK employer and an overseas employer and the overseas employer:
Earnings are not aggregated and NICs are calculated separately for the earnings from each company if an employee receives earnings from separate employments with a UK employer and an overseas employer and the overseas employer.
If the overseas employer does not have a place of business in the UK, check whether the UK employer is liable to pay NICs on the employee’s total earnings (from both the UK and overseas employer) as a host employer. See NIM33023.