As I understand it, under Directive 2004/38/EC, an EU national who has only ever lived in their own country is governed by national rules when it comes to inviting non-eu family members to join them. However, as soon as you've moved elsewhere in the EU to work or study, and thus "activated" your Treaty Rights, the rules covering you on non-EU family members are the EU rules.
As I've understood from things like this and this, the Surinder Singh case ruled that once you've activated your Treaty Rights, if you move back to your country of citizenship, you remain covered by the EU rules, and you're no longer subject to the stricter national ones.
Something like a short holiday in France isn't enough to activate your Treaty Rights, so presumably just rocking up at the UK border from abroad isn't enough, and you'll have to show something to demonstrate to UK Immigration Officers that you have been exercising your Treaty Rights.
So, what would a UK citizen who's been living + working elsewhere in the EU need to bring with them, to prove to UK Immigration that their non-EU family members can be admitted under the EU rules?