If you are leaving the UK to work abroad for at least one year (or permanently), there is a requirement to notify HMRC. This is done using a P85 form which should be completed and submitted to HMRC. You will also be required to submit a Self-Assessment tax return if you usually complete one, for example, if you are self-employed.

The completion of the P85 form will also ensure you can claim any tax refund you are entitled to and will also help HMRC decide how you should be treated for the purposes of UK tax. 

The P85 form can be found on the GOV.UK website. There is an online form available as well as a postal form. Your liability to Income Tax will depend on whether you are resident and / or ordinarily resident and / or domiciled in the UK. 

It is also important to consider the National Insurance (NI) implications of working abroad. In some cases, it can be beneficial to continue paying NI contributions whilst abroad as this should help you secure state pension credits.