How might you do that as a complainant? There's no dismissal by oldco, since what happens is that an employee resigns, newco applies for reference, a 'discriminatory' (do you mean this v-a-v a protected characteristic, or are you using this in a layperson's sense?) reference is given, and the newco dismisses in good faith, based on that?
Reproduced from an Employment Lawyers' site, it covers the two main forms of redress:
Can you sue someone for giving you a bad reference?
If an employer gives an inaccurate or negligent job reference, the employee can sue their former employer to recover damages. This claim is brought in the County Court NOT the Employment Tribunal. The former employee must have suffered financial loss ie. that the ex-employee failed to get a new job because of the bad reference and lose the income that new job would have provided. Legal Aid may be available to assist you to pursue such a claim.
Defamation claims, when it comes to bad employee references, are quite common. However, they are also difficult to prove and largely depend on the circumstances surrounding the claim. It must be proven that the employer provided a poor or false reference with malicious intent. If the reference was provided without malice and the employer believed the contents of the reference to be true (even if they are untrue), then they can’t be successfully sued for defamation.
You can go to an employment tribunal, sadly there are many examples of which you are oblivious and deliberately tuning out of your consciousness (for whatever reason, I'm not sure). You are simply not even wrong on this point. It is a trivial and very serious point that we're talking about protected characteristics.
Unfortunately you're also rather lazy, since I outlined the difference between negligence and defamation in this sense above and you thought nope you're a clever sod so I'll google it, copied and pasted the first result (cheeky) which doesn't even explain things fully and goes over the points I made. I've read many employment sites and an employment textbook on the matter. Try better googling in future.