I know several of the staff at GWHQ in Nottingham had what was basically their own separate ruleset cobbled together from what they considered the best parts of the games over the prior 25 years. Most clubs had enough house rules that they were basically playing separate games. Although with the current player-unfriendly stance any ruleset distributed as "for GW figures" would find its self on the wrong side of a legal team.

And then a lot of players have very, very irrational attitudes towards proxy figures. I couldn't turn up with a Warmachine army sheet and my Imperial Guard army and play, or vice versa. In fact, that'd get you kicked out of a lot of hobby shops.

There are generic wargame rulesets out there that you can play with basically any minis. For instance:

http://15mm.co.uk/USE_ME_15mm_Sci-fi_wargame_rules.htm