As explained in the Heathrow Customs - “Arrivals from the EU” exit question, the UK is somewhat unusual in having 3 customs exits from the baggage hall. There's a Red one for something to declare, a Green one for nothing to declare, and a Blue one for Arrivals from the EU
On an upcoming trip, I'm flying into Europe from outside the EU, and have a fairly long layover in Amsterdam before continuing on to the UK. During this layover, I will probably leave my bags checked through, but I'll enter the Netherlands through immigration + customs and spend some time there. Later I'll return to the airport, go through the Schengen exit immigration (+security), and take my flight to the UK.
In this situation, I as a person will be arriving from another EU country, having cleared customs and immigration there with my hand luggage. However, my checked luggage will not have entered the EU yet.
When I arrive in Heathrow, I'll then have an odd mix. Myself and my hand luggage are an EU arrival, checked luggage is not.
What queue should I use at Heathrow customs in this situation?