Millarworld's worth as of the last accounts submitted was about £2m, most of that apparently in the form of assets - most likely, promises to pay on completion of a project, meaning that Millarworld could be said to be *worth* those assets only in the sense that it could borrow against them as equity.
This would only be advisable if Millar or his second wife (the sole directors of Millarworld) were certain that they would be able to repay any loan, meaning they'd have to be pretty certain the projects were going to complete (otherwise they'd be stuck with the incremental payments for various stages of completion, and never receive the full amount). Accounts filed suggest they've taken money out of the company, but were unable to leverage the full worth (until now).
Millar's other company (that could possibly be part of this deal) Dave and Eggsy Ltd, is worth a couple of thousand £ and probably is simply a vehicle to avoid paying income tax on his writing work, so unlikely to be included. Millarworld Kids was dissolved over four years ago, Mark Millar Ltd the year before that, and Clint Entertainment Ltd is apparently dormant, having been declared dormant two years ago, and so no longer trading. It's probably worth about £60k today.
While it's possible he has other companies overseas that aren't required to file accounts or register themselves in the UK (as would be the case if they never traded in the UK or routed any transactions that way) it sounds like Millarworld's sale is Mark Millar cashing out ahead of his retirement; possibly in a creative role (again, the company has only listed two directors, Millar and his second wife, and also files total exemption small company accounts, meaning it likely has no other employees, temporary or otherwise). And hopefully he's got his full £2m, cash, and a company to receive it in (otherwise he's going to end up with about £1.1m, because that's a hell of an income).