We have a flat. What's next? Register now or later?

If you are British citizens, you do not need a work permit. You really should do what the rules say, if you wish to exercise your treaty rights, go and register yourselves, you'll be given 3 months to actually exercise your rights by studying, working, investing, being self sufficient. If after 3 months, you don't have jobs, well as Brit cits, you'll just get a notification you are not registered. No-one with EU nationality is removed forcibly unless they are a threat to public good, basically if you have a criminal record, or are terrorists, that sort of undesirable person.  It will actually be in your own interests to register. If you've worked immediately before leaving the UK, you should have brought with you form S1 to register with a health insurer and get immediate coverage too.