I think how it gets prosecuted also depends on if there is a way to prove collusion between the two companies in this. It is absolutely fishy that Epic laid everyone off just before the sale/transfer concluded to Songtradr and both companies can currently pretend they did nothing wrong and hurting the union was an accident of bad timing and Songtradr especially looking "clean" hiring back in "waves" based on BS metrics and "goodwill" since it could have just not offered jobs back to people already laid off by the previous owner. It can be hard not to imagine that there wasn't some "golf course handshake" on the whole thing, but proving that existed may be tough to do, especially if the collusion was literal golf course handshakes with no paper trail.
This should be illegal but it just sounds like good free market business.
If they were Epic, not Bandcamp, employees then Songtradr would be within their rights to proceed as they have. But Epic would have had to go through a redundancy process (including explaining why it's happening, detailing the alternative tactics they have explored and trying to find other roles within the organisation for the affected employees).
Of course, it's all immaterial as UK law doesn't apply (and I only took a short qualification in it so I know it's fiendishly complex anyway).
EDITED to remove information I had included in the original post
The new company has to offer employees a "transfer of previous employment" - so even though you're working for someone else, your history at the previous company is moved over with you. So if they wanted to get rid of you, they would then have to go through a redundancy process - which involves explaining why they're letting you go, offering you alternatives and giving you a payout based on your length of service.