As Hat have announced figures to do this period in addition to Emhar (see below) I thought maybe I ought to look and see what rules are out there. These Days of Iron look good. And they're free. If you want to spend money this set of rules 'They Died For Glory' also look good.
So far the only company I can find producing white metal figures in 20mm is B & B sold through Brookhurst Hobbies in the US. They seem pretty comprehensive though as to whether they mix or not with 1/72 we shall have to see. c/o Mac Warren 184 Woodcock Street Hessle Road Hull HU3 5DW