Firefly (WW2) and Challenger (Modern) rule-sets by TTG replaced WRG in the 80s.
Command decision I think I saw played by a few folk at the club, but it's a US import so still not cheap then (90s?). Also, it was not a satisfying scale (1 vehicle=platoon, not 1 to 1). Plus, all the inter-club and the nationals and the world team used WRG, then TTG rules (WRG rules had lost the plot with their later editions espousing rather convoluted and obtuse command and control rules, and thus losing the simplicity of play the originals had, TTG rules were playable for large battles).
25mm then 15mm Ancients of course were strictly WRG rules from 1st edition through DBM etc.
I have not looked at the Derby site for a decade or more - but here is their rules page for this year (it is unclear what modern 6mm rules are in use from that).
http://www.worldwargames.co.uk/
The World Teams at Derby got players from all over - I think I fought American, French, Dutch, South African players as well as UK & Irish ones over the years (modern and ancients). Though now it seems to be individual entries rather than 3-player teams (in the 80s and 90s it was held in a big conference and snooker centre in the middle of the town - now seems to be in the Uni campus?). But I last went down in about 97 or so.
Cheers
Andy