At many (most?) EU airports, some items in the duty free shop show different prices for people flying to EU vs non-EU destinations, and some items simply aren't for sale to people flying within the EU.
I'll be flying from a big EU airport with a good duty free selection, onwards to another EU airport with I think a less good one, and finally connecting onto a a flight out of the EU. I'll have boarding passes for both legs.
At my starting airport, am I able to buy "non-EU duty free" by showing both boarding passes to the staff in the duty free shop? Or would I be restricted to only buying the "non-EU duty free" items before the second flight, the one that actually has me leaving the EU?
(Hopefully there is an EU-wide rule about this, if not, I'm specifically interested in the case of departing LHR for another EU airport, to connect to a flight out of the EU)