I'm surprised that this hasn't been mentioned, but if there are rules and fines then there must be some kind of regulator.
You need to contact that regulator - anonymous, at least at first, and ask their advice.
You also need to call ACAS now http://www.acas.org.uk
Although the top rated answer is fine (Contact a lawyer), I suspect that may well be easier said than done. Most solicitors are either criminal defence, specialise in something like family law or conveyancing or tend to be corporate focussed. I'm certainly not implying it's impossible, but I think the average "man on the street" will struggle to find a legal expert with the knowledge and inclination to get involved here.
With all that said - I'm presuming whatever it is you're discussing is either criminal or morally outrageous. I'm not saying it's fine, but I certainly wouldn't go full Snowdon over a company breaking some kind of contract with another company. It's certainly indicative of their corporate personality and I'd probably leave, but some scale is needed here and some fights simply aren't yours to worry about.