I'm in germany and I have a friend who lives here with his family, but they are still british citizens. So, from our side of the channel: Pretty much everyone in their situation has been contacted by the german government - temporary allowances of residences are prepared for this situation for everyone in their families. All they have to do after an exit is to go to their local citizens bureau in order to sign and collect the papers. After that, they are fine until a more permanent solution is found.

However, to answer your question: Coming back from abroad is a complicated thing, even from France inside the EU. Technically, after a hard brexit and without these temporary papers, they are citizens of a foreign nation and the german state has no diplomatic relationship with this state. As such, they could get detained or denied entry if they run into some kind of border control. As such, it is recommended to not leave the country while this is going on. This had tossed quite the wrench into their vacation plans.

You'll need to have applied for settled/pre-settled status, I'd imagine, unless an Irish citizen. And then ask the immigration officer to check gov.uk for you, because there's no actual legal paper or ID issued that you can carry with you.

> 10. Until 31 December 2020, EU citizens will be able to enter the UK by showing either a valid national identity card or a passport.