EIM34160 | Foreign travel: approach to be taken on residence status
From HM Revenue & Customs · Employment Income Manual
The foreign travel rules at EIM34010 onwards apply only to employees who are resident in the United Kingdom. Some deductions are prohibited if the employment is with a foreign employer and the employee is non-resident or a qualifying new resident in the United Kingdom. This requires consideration of:
the residence status of the individual
the state in which the employer is resident.
Unless the employee has completed the non-residents pages which accompany the Self Assessment tax return, HMRC may accept that an employee is resident in the United Kingdom for the purposes of the foreign travel rules.