I think that in your stated areas of interest (WWI, WWII and on to the present) your best choices are as follows:
WWII to the present: 1/30 scale (60mm) figures, vehicles and diorama materials by K&C, Honour Bound and New Model Army. These companies all produce substantial WWII ranges.
K&C is really the only company to produce substantial modern war (Vietnam, Gulf War, Afghanistan and Iraq) metal figures, with complimentary armored vehicles [M113 APC, Duster from Vietnam, HumVee and hull down T72 Tank from Iraq & Afghanistan] aircraft [Huey Cobra and Kiowa helicoptors and F4 Phantom and Mig Aircraft from Vietnam, F14,F15, F16, Harrier, & several other modern jets from the Gulf War through today] and diorama materials [Vietnam Firebase, Tet Offensive Street Scene, Afghanistan special forces mountaintop diorama].
Figarti also produces excellent WWII, Korean War, Vietnam and Modern Iraq ranges, with absolutely superior vehicles, but in 1/32 scale.
As far as WWI is concerned, Trophy, Toy Army Work Shop, & Tommy Atkins all make very nice traditional glossy toy soldiers, vehicles and diorama pieces in 1/32 scale (54mm). NWTC and Frontline have done some WWI in matt, and I believe K&C intends to do some in the near future. I hope this helps.