People from the EU never have to declare things (I think?).
No, you've got to declare, inter alia, currency over, IIRC, £10k (might be €10k), and anything that isn't a 'personal import' - so goods for resale, probably commercial samples, that kind of thing. (Steph would know, I guess.)
I go through which ever one is least crowded
Is probably the right answer.
I don't know if you canmay go through the blue channel just because you've travelled from an EU country, or if you also have to be an EU citizen/resident.
All wrong. Kim was closest
People arriving from the EU do not have to make a declaration, full stop, because that is the law. It is a Customs control, not a passport control. Nationality and residency are irrelevant. Commercial goods are dealt with through the VAT system, and you do NOT have to declare currency. That (and it's Euros10k) only applies to people leaving or entering the EU at an external frontier. Fly into Bristol from Spain, no declaration. Fly into Bristol from Brazil, declaration. Fly into Bristol from Brazil having transited Spain and not 'landed' (cleared Customs), declaration. The same outwards.
As for prohibited or restricted goods (weapons, drugs etc) the offence is committed as soon as the item enters UK airspace/territorial waters, so there is no question of declaration.
Simple rule: you can only use the Blue if you have arrived from an EU country. You do not HAVE to use the Blue if the Green is easier or less crowded. There are, of course, massive posters explaining all that all over the baggage reclaim area. In pictures for the hard of thinking.