Those hangars you speak of are in use though, aren't they? For the fire service, maintenance, de-icing equipment etc I agree there is no need to acquire more land right now, they have a lot to play with. The focus needs to be on improving and greatly expanding what they have, not gaining more land.
A big expansion of the SE Pier with new widebody stands is about the only way they could go. Start off by building a new area and slowly link it back to the main terminal, improving the current pier next and then working back towards the main terminal, which would really have to be demolished and rebuilt bit by bit. With a new terminal operating where the SE Pier currently is, they would probably just about be able to get away with using existing stands on the main terminal building as remote by coaching from the new and improved terminal for a few years and eventually the whole building could be up to the same, modern standard. New check in halls are needed, not just pop up machines and moving bags around in wheeled cages.
I believe the portacabins are used solely by the border force, possibly as admin offices or temporary holding cells until the people can be allowed entry or sent back. The fact there are still portacabins there is a bit of a shambles and in all honesty I'd rather see that whole area turned into a terminal extension with proper gate rooms and jetty served stands which feed into a bigger immigration hall. That would negate the need for that stupid corridor used for international arrivals at present, which causes more harm than good. The portacabins could be relocated elsewhere in one of the other areas where the airport grounds have lots of unused space. I have a feeling though that border force would veto any attempt to remove those portacabins, sadly.