I'm an American who was retaliated against in the past for collective bargaining efforts. Luckily, that's illegal here as codified by the National Labor Relations Act of 1935 (it probably is in the UK too, I'm just not as familiar with their laws). I filed a complaint with the National Labor Relations Board (NLRB) and eventually won my case, receiving compensation with interest; the Company also had to inform all employees of their collective bargaining rights digitally and physically.
Once the government shutdown ends, I highly recommend the affected American individuals file a complaint with the NLRB via their website: https://www.nlrb.gov/
I should have mentioned that the government foots the legal bill! That is, you don't need to hire your own costly lawyer. The case will eventually be USA vs. (your old company), and you're a potential beneficiary.
Once the government shutdown ends, I highly recommend the affected American individuals file a complaint with the NLRB via their website: https://www.nlrb.gov/