I've never had anything like this happen to me in an academic environment, so I'm sorry you had to endure that (I have had racist abuse, just not within a University setting). If they were smart enough to get into Uni, you'd think they'd at least have the common sense to keep their mouth shut about that sort of thing.
I'm also a bit surprised that the conduct team advised you to report it to the police straight away, as i would have expected them to try and deal with the incident themselves (I mean internally, there must be a policy for this kind of thing?)
Why do you think they heads would coerce you into retracting your statement? Is it that they wouldn't want the publicity; is the PhD student very popular / influential? Or is it simply worry / paranoia? Whether or not you're on probation is completely irrelevant and shouldn't make a difference. If you're a victim of a crime, then you're a victim; end of!
Whether or not it could be considered as "
perverting the course of justice" depends on what they threaten you with, and again whether or not you can prove it (in the UK, you are innocent until
proven guilty) f. If it's a whole department behind this, they may know how to cover their tracks; especially if they've had similar situations before.
It depends on how much evidence you've got, what exactly was said to you, and the surrounding circumstances (i.e. was it "
out of the blue" or the climax of a long-standing tension between you and this other person). If you can prove what was said (e.g. you have a recording or independent witnesses), then it MIGHT lead to something. However, if you don't have any of the above, then realistically all they can do is invite this person for an interview to give their side of the story, and it'll be documented and that will be that (if they admit it, then they may receive a police caution).
You don't have to go into detail, but the link below gives you a guide as to what you can plausibly expect, if the police & CPS were to pursue your complaint to the full extent of the law.
https://www.sentencingcouncil.org.uk/offences/magistrates-court/item/harassment-stalking-racially-or-religiously-aggravated-harassment-stalking/