Based on the Withdrawal Agreement, Hungarian nationals are able to work in the United Kingdom without a work permit during the transition period (until 31st December 2020). Their legal status matches those with the right to free movement and residence, however, they are obliged to get themselves registered with the UK authorities before 30th June 2021.
There will be a visa requirement for Hungarian nationals who will arrive in the United Kingdom after 1st January 2021 and want to take up employment. Please read the British government’s visa guidance for EU citizens carefully (available in English and Hungarian).