Malt is usually made of mixed amounts of varieties.
A friend of mine who works in Soufflet malting plant said it's strange that in UK so old varieties are still used, like Optic, Golden Promise or MO, they're now over 30 years old at best. In continental Europe varieties have much shorter "life", it's usually 8 years and then they are replaced by newer, better suited, less vulnerable. As I remember, 2-3 years ago the most sought variety was Sebastian, now it's nearly extinct and replaced by newer varieties.