At Singapore Changi airport, arriving and departing passengers happily mingle.
The idea behind segregation of arrivals and departures is twofold. One of or both apply at all airports.
1) So that arriving passengers (potentially from airports with less stringent security) cannot hand prohibited items to departing passengers.
2)So Passengers cannot exchange documentation to let people enter/depart the country who shouldn't.
To tackle this, security can be done in a few ways- total segregation of the two flows through the whole journey. T5 does this with departing passengers passed through security early in the journey (but allows international and domestic departing to mix due to the photo process).
Many EU airports ie Madrid, Barcelona, and indeed outside EU, Singapore operate a different system. They do allow departures and arrivals to mix but security screening is done at the gate, so if an arriving passenger hands something to a departing passenger, it will be picked up at the gate.
There is an argument that it is a little egotistic for the UK to assume that all other countries security is worse than ours, which is why arrival passengers aren't "trusted" to not have illicit items (not just talking illegal, but lighters, knives etc). Recently there is a movement around the patch that says there are a number of EU countries whose security can be "trusted" and for whom therefore, such segregation would not be needed. It is being looked at for how to improve transfers by removing the need for transfer screening, but because space it limited in most airports, it would be hard to implement a Domestic and EU process and a separate "other" international process.
Watch this space i guess!!