Basic Rule - You pay contributions in the country where you work
If you work in a country with which the UK has a Reciprocal Agreement or Double Contribution Convention you are usually insured under the social security laws of the country you work in. You will not usually have to pay UK National Insurance contributions. But this is not always the case.
Working in Social Security agreement countries
If you go to a country with a Reciprocal Agreement or Double Contribution Convention (known as Social Security agreements) and start working for an employer there, you’ll usually pay social security contributions in that country rather than UK National Insurance contributions.
These countries are:
Barbados, Bermuda, Bosnia-Herzegovina, Canada, Croatia, Guernsey, Israel, Jamaica, Japan, Jersey, the former Yugoslav republic of Macedonia, Mauritius, Montenegro, New Zealand, Philippines, Republic of Korea, Serbia, Turkey, USA.
Most of these agreements have a provision for someone who is in business for themselves in the UK to continue that business in the other country whilst remaining UK insured.
You should apply to obtain a certificate of continuing liability from HMRC Residency (Newcastle) to prevent a demand for the payment of contributions in the other country.
You will have to pay Class 2 and possibly Class 4 NI contributions as if you were still in the UK. Providing you have a certificate from HMRC Residency (Newcastle) you will not have to pay into the social security scheme of the other Reciprocal Agreement or Double Contribution Convention country.