My guess is that the Dan Dare Corp (and some of the other IP owners who seem to just be sitting on material) refuse to accept a lot of (what seem to be reasonable) offers because if they did accept they'd be setting a price below what they've valued their IP at.
They'd rather bring in zero income and claim it's worth X million pounds than make at least some money but have to admit that they previously over-estimated the value. I'm assuming there's a tax advantage or similar involved somewhere along the line.
if you look at the Companies House info on the rather grandly-titled Dan Dare Corporation, you see its only two personnel have the same surname (husband and wife team, I'm guessing) and that the company address is a private residence in a very leafy part of Surrey.
There's little to no money actually moving through the company, and. yeah, the impression I get is someone just sitting on their IP in the fading hope it might be worth something again as a film or TV property.