Hi, I would echo the view that in 15mm Peter Pig's range is pretty comprehensive, especially if you really want to do early war. You can even buy Belgians! In 10mm Pendraken are hard to beat but of course the detail isn't quite the same, albeit the figures are only about 1/3 of the price. I've noticed that Plastic Soldier Company sell bags of 150 15mm British or Germans from their "Great War" boardgame. They are late WW1 but amazing value at what seems to be £7.50 per bag.
I'd check out the various websites