The Jackson Gamers have this set of free wargamesrules for the English Civil War. They use a card activation system, (not surprising, considering the acknowledged Brom influence), and d6s to resolve actions. These rules are 20 years old and in their 6th edition, so they are well-tested.
english civil war
Initiative Cards For English Civil War
The Lead Gardens Blog offers a set of iniative cards for English Civil War games. They’re designed to work with the Victory Without Quarter rules, but no doubt could be used for many others.