Pretty much as the title; I've looked at vassal 40k, but it hasn't been updated in a while and is missing units. I'm not necessarily looking for something for WH40k in specific - as long as it allows for moving units, measuring distances, and custom sprites/enough sprites to use "counts as" we should be fine.
As @Skeith mentioned, Games Workshop LLC is very vicious when it comes to protecting their Intellectual Property license, so it will be very hard to find anything designed for this on purpose.
That being said, Steam recently released a game called Tabletop Simulator, which, with a lot of work, could probably be used as a tool for this (right up until GW catches on and their lawyers destroy the life of whoever is supporting the mod).
Gw are famous for taking legal action against anyone and anything that uses its IP without license.
Vassal were the last to try what you are asking and i believe were shut down with a cease and desist.
Sadly anything close to what your seeking would be shut down quickly by GW. This means no one is making any such software.
The closest program that I can suggest for doing this is with Tabletop Simulator. It is a program on Steam that is a physics engine with a wide variety of pre-loaded boardgames and tens of tools that allow you to emulate virtually any table-top game imaginable. In fact, the game has it's own Workshop application that has tens of thousands of games, sets and models to choose from, including some from WH40k...
You could use roll20.net Whilst geared towards RPG, it does come with a tabletop with grids or hexes, you can define the size of each grid, u can place custom tokens and what not, but you would need to do a lot of the prep work yourself in terms of uploading images for the units and preparing the map
I know this sounds simple and slightly low-tech, but setting up a board with proxied models and emailing turns is always a good way to play. I am working on something to simulate games, and am going to try and get it published with Games Workshop, but even if that does work, it won't be as good as actually having the table set up in front of you.
VASSAL works and there is a 40K module for 7th edition.