If your employee remains resident in the UK, as a UK employer you will still be liable to deduct tax and NI contributions, even though all or most of the duties are performed abroad.
If the employee becomes non-resident, you are no longer required to deduct tax and NI contributions. However, you will have to take advice as to whether local tax is payable.
If any income is taxed in both countries, then double taxation relief will apply, with any foreign tax paid being set off against the individual's UK tax liability.
