the new residence requirement came into force from January 1, 2019, and according to the new rules the A-kasse can pay you unemployment benefits for 6 months, while the A-kasse collect documentation for your stay abroad in a country in the EU, Norway, Iceland, Liechtenstein or Switzerland (via forms to foreign authorities).
However this only applies if the A-kasse member can provide (other) unambiquous evidence of the stay in an other of the above mentioned countries. In your case from your stay in the UK. I would say a tenant-agreement could be a good proof.
However we can not say "what is normal" in this regard. The residence requirement has only been in effect for 6 months, and we need to see how the A-kasser will practise the new rule. And there will probably also be some complain cases that will tell us and others how the A-kasser must process the new requirement, for example about paying unemployment benefits before your stay in another EU-country is confirmed by the foreign authorities.
So therefore I am not able to (at the moment) to say would should be considered as enough proof/evidence.
If you can not provide this other evidence/proof that satisfies 3F A-kasse, then your only chance would be to apply for an exemption from the Danish Agency Labor and recruitment (Star.dk) for payment of unemployment benefits in the "waiting period".
Anders Weber, www.a-kasser.dk