Thank you for your enquiry.
First of all EU nationals and their family members have the right to move freely and reside within the EU but a British national and his/her third country national family member can only benefit from free movement rights in the UK if they meet the criteria under the Surinder Singh principle.
So if a UK citizen is exercising their treaty rights in another EU member state, they should be allowed into the UK with their family members on the basis of their entitlement to free movement and should therefore not require leave to enter or remain in the UK. A British citizen under the Surinder Singh principle and their family members should be dealt with as persons seeking admission in accordance with EEA regulations and not as persons seeking leave in accordance with UK domestic immigration laws.
Non EEA family members are usually informed to obtain a EEA family permit in the destination EU member country to avoid any issues. Where there is no family permit I would expect a third country national seeking admission at port as a family member of an EEA national exercising treaty rights to be assessed whether or not they qualify for admission under EEA regulations. So you should ensure you have all supporting documents.