The original T2 concept was based on a linear approach prior to 9/11 with a significantly lower security threshold and tolerance with all gates being contact and directly in front of the customer.
It was intentionally designed with a low dwell time kerb to gate principle in mind and it worked until government imposed border changes and differing security regulations on the industry including sterile separation of arriving and departing passengers (both having be security vetted somewhere)
The original T2 was linear but also had remote gates opposite. The original intention was to eventually build another small pier in the grass island opposite - there was a considerable debate whether to build a subway under the apron for this as part of the original design to minimise apron disruption and activate it when this pier was built. In the end the cost was high and it wasn't built - a sensible decision especially in the light of hindsight.
T2 was built from day one on the principle of segregating inbound and outbound passengers. One of the very first meetings during the design phase was with the DfT who made it quite clear that they would not certificate the terminal if this principle wasn't adopted. This was achieved by having departures above arrivals and passengers never had to go up, it was always down for the normal flow. Of course this principle was abandoned in legacy T2 some while ago for departures. In addition, the idea of having one large departure concourse airside meant that most passengers could remain in this area for a while and spend money....
There was a change to the design of T2 security at a late stage. This was down to the requirement to incorporate hold baggage screening after Lockerbie. There was quite a cost for that which the Airport had to pick up. It was an interesting meeting, that one.
Meanwhile existing facilities at UK airports were able to take advantage of their grandfathered status of allowing the mixing of inbound and outbound passengers. That ended abruptly after 9/11 and the timescales for making alterations was very short, hence the arrangements that now exist in T1.