Just modify the law so that databases of postal addresses are not copyrightable.
Not sure that's a precedent I'd want set in a common-law country, and not sure that would hold up to judicial review under common law.
The government made a bone-headed mistake when they included the postal data as an asset in the sale. The solution is for them to admit their mistake and pay for it. It's fiat money anyway, so it doesn't really cost anything. Having them abuse their government power to cover up their mistake is not an approach I endorse.
Not that this hasn't happened before, think postal scandal or yesterday's comments on the Hawke and Curacoa https://news.ycombinator.com/item?id=41285275
In any case, nationalisation has a long history in the UK, so it would hardly be setting a precedent.
One way or the other, a private asset must be either nationalised or compelled to be released.
Gradual renationalisation of the rail network was in the manifesto. That's not particularly contentious, as rail franchises have fixed terms. But the manifesto is all about steadying the ship, and militant nationalisation risks spooking investors, so whether the government has any appetite to nationalise anything by fiat is questionable.
Nonetheless, there's public support for renationalisation; and, for such a low-value asset, this might be a nice test of the waters.