Check out StarSIEGE by Troll Lord Games.  It's an Role Playing Game set up specifically to allow for customization by the GM.

Well, mechanics-wise, I don't think there's anything on the market (yet ) that is similar to Iron Stars.

But setting/universe-wise, I can think of several:

Space 1889
Castle Falkenstein
For Faerie, Queen and Country
Victoriana
Age of Empire

Although these fall into the Victorian era, and not the Edwardian era of Iron Stars, they are far and away the best fits of the available games.

Forgotten Futures would be great.  It already has the "feel" and is free.  It is well supported  (10 "setting" books already) and should give you alot of great ideas to use in IS.  I don't use the mechanics but the setting ideas are great.

I play Savage Worlds.  It's a tabletop miniatures game with RPG overtones (strong overtones).  The great thing is the rules can be tweaked to your heart's content to get the right ambiance for your game.  Fantasy to SciFi is covered. It's simple but not simplistic.  You can order the core rules as a PDF and I think its about 10 USD.

I actually use IS to play out space combat in my present Savage Worlds campaign.  I have to make some additions to the IS rules to take PC actions into account, but the changes are very simple and straightforward to implement.

Compared to a regular RPG space battle IS space battles are extremely quick and merciless...you have been warned.

Hero System is specifically designed to allow you to play in any genre.  The basic game has been around since '81.   It is not designed to play a setting out of the box.  But rather so the GM can set up the setting he wishes. 

Good system.