As of January the 1st, 2021, as a result of Brexit, new rules for employing British citizens in Poland apply.

If the British has not retained his right to reside and freely undertake work in our country on the basis of the Agreement on the withdrawal of the United Kingdom of Great Britain and Northern Ireland from the European Union and the European Atomic Energy Community (the Withdrawal Agreement), employment requires the same rules as for non-EU nationals. It is connected with the necessity to obtain a work permit by the employer, as well as the obligation to respect the limit of the permitted period of stay under the visa-free regime in Poland or obtaining a visa or a temporary residence permit by British citizens.

Moreover, the requirement to go through the work or residence permit procedures for Britons results in additional obligations for employers. If they obtain a work permit for a UK citizen, they will have to comply with the obligations that come with this type of permit. In the case of visa or residence permit proceedings, employers will have to provide British nationals with the appropriate documents confirming the intention of hiring and working conditions.

It also becomes more difficult to delegate a UK citizen to other EU countries. It will require obtaining appropriate permits authorizing the British - also the beneficiary of the Withdrawal Agreement - to stay and work in the given country. The rules for ensuring appropriate employment conditions for posted workers remain unchanged.

Social insuarance issues are regulated in the Trade and Cooperation Agreement between the European Union and the Atomic Energy Community and the United Kingdom of Great Britain and Northern Ireland. According to the arrangements, contributions are, in principle, to be paid in the country of work. However, in the case of employees posted to Great Britain, under the conditions specified in the Agreement - in the country of employment. In relation to the British posted from Poland to another EU country, the current provisions on social insurance apply.

Beneficiaries of the Withdrawal Agreement are entitled to freely take up employment, however, by the 31st of December 2021, they must replace their residence permits with new ones, confirming their new status.
 

More in the article by Michał Kacprzyk and Marta Dudczak "Employment of British citizens under new rules" for Rzeczpospolita (article from 21 January 2021).