Or whoever they transfer their address space to, if they try to be more socially responsible, which might end up being the next cool startup.
That would be socially irresponsible. Use IPv6.
I suspect major cloud providers may well end up buying the address space if it were to become available, so quite a few companies' services may not be reachable.
That's the best case scenario, considering that most customers probably don't need to connect to UK DOD. With the way IPv4 shortages are going, the more likely scenario is that some cloud provider/cdn buys that range, and sites start breaking randomly for customers.
Perhaps. They sold a block to Microsoft to host O365 and Azure AD components a few years ago!