While bullying itself is not strictly illegal, it usually has an underlying cause which is. If the bullying is related to age, sex, disability, gender reassignment, sexuality, race or religion, then there is a legal case to be built. If you think you're the victim of a bullying campaign, you need to start collecting your evidence to back up your claims.
Examples of bullying might include regularly being undermined in front of colleagues, constantly being overlooked for promotion or training, people spreading malicious rumours, or just generally unfair treatment. It could be committed by one person or many, and might take place face-to-face, by email or by phone.
Whatever form the bullying takes, it can have a damaging impact on your emotional health and you need to take action. In order to build a case you can take to your HR department, you need to collect evidence of occasions when you have felt you were victimised. Start saving any intimidating emails, get supportive colleagues to provide statements attesting to things they have seen and keep a journal recording events, so you don't forget the details.
Remember, bullying is never the fault of the victim. You've done nothing to provoke the behaviour, and workplace bullies usually behave in this way because they're preoccupied with climbing the career ladder and feeling powerful. Some managers may pass off genuine cases of bullying as a 'clash of personalities'. Yes, personalities may clash, but however bad the situation, it shouldn't never entail physical or verbal abuse.
If it's your manager who is acting the bully, don't let them get away with describing it as their 'leadership style'. Nobody should lead by being dominant, aggressive or intimidating. Your workplace has a duty of care to protect you from bullying, no matter who it's coming from.
Bullying isn't illegal, as I said, but discrimination and harassment are. If your HR department can't help, it's time to seek legal advice and consider a tribunal hearing.
Bullying is not illegal, discrimination and harassment are
Bullying can take many shapes and forms
Start collecting evidence to support your case
Bullying is never the victim's fault
Consider legal advice and a tribunal case