I'll also chime in and say go Vallejo. Coverage and color pallette is superb. However, if all you have is GW paints, then here's what I'd do:
British
Uniform-Graveyard Earth
Rifle, entrenching tool-Scorched Brown
Metal-Boltgun Metal
Helmet-Catachan Green
Wegging and equipment pouches-Khommando Khaki
Flesh-Tallarn Flesh
Boots-Chaos Black
German (tougher IMO)
Uniform-Catachan Green
Belt, suspenders, boots-Chaos Black
Canteen, tool handle-Scorched Brown
Tent Half-Bestial Brown
SMG mag pouches-Khommando Khaki
Guns and metal-see above
Helmet, gas mask cannister-the darkest gray they make, whatever it is
Three color camo-Desert Yellow, Snot Green, Bestial Brown