I'm not sure I can see the need for those giant metal case unless to are transporting an apocalypse army or are fighting Waterloo regularly. A 2000 point force will easily fit in two cases with room to spare.
I have about 4 card cases and one metal case and swap the trays in the card case in to the metal as required. It's the best storage system space wise. I also have their double backpack but haven't used it yet.
I'll even stick my neck out and say you will be fine even with just card cases. In London on the tube/buses, card will be okay as well.
The standard KR trays suit
GW infantry perfectly. That said if you have spears or Ogres then this may well not apply.
Not quite the same but I have one with
FoW models in it and I rather foolishly order the wrong tray and didn't have space for my large tanks. 10 minutes with a sharp knife and a hot glue gun solved that. That said the people at KR know the hobby and models and are best suited to offer advice on the right trays. They are also thoroughly nice, which helps.