The rules stating that arriving and departing passengers cannot mix are set by the DfT and also UK Customs. The idea behind it is that the DfT don't want passengers who have arrived from countries where they deem security not be to UK standard mixing with departing passengers travelling on UK airlines. It is also a Customs regulation as Tax Free products could be transferred between departing and arriving passengers. These regulations are set by UK not European authorities and that is why such segregation may not occur in other countries.
The way the Piers in T1 were designed meant that MA had no choice other than implement the current system for seperating the 2 sets of passengers. T2 & T3 have no such problems as arrivals and departures are on 2 different levels. The situation is by no means ideal, particularly on B Pier but when the regulations were set the airport adhered to them as quickly and as cost effectively as possible. It is obvious though that the current system is far from efficient in terms of ease for passengers and now the airport is looking at knocking down and rebuilding B Pier as well as remodelling C Pier. However, this in itself creates the problem of where to park aircraft whilst the piers are being redeveloped. Not an easy task and the current mish-mash of walkways will be around for at least another couple of years.