1. I hesitate to differ from barmaid (who is, after all, a barrister) but this is a civil matter - not criminal - so any warrant would come from the County Court, not magistrates. Before that stage is reached, a Court claim will have to be issued. Your friend can then dispute the claim and deny she owes the money. She can provide evidence to Court & - hopefully - convince the judge that she does not owe the money. If the judge is not convinced, he/she will grant a County Court Judgement, which may say the money is to be paid immediately or in instalments over a period of time. If the terms of the Judgement are kept to bailiffs cannot be instructed. So you can see any question of bailiffs is a long way down the track, & there would be a good opportunity to make the case in Court before they can be involved.
2. These are undoubtedly bully boy tactics, intended to frighten. Don't let them succeed. If anyone does visit the premises before the above procedure has been carried out they will not be bailiffs. They may pretend they are, but they are not & have no powers at all. They should not be admitted, but told the debt is disputed and to leave.
3. If it does get to the stage where bailiffs could come, make sure all doors & windows are kept shut. Don't admit them - they can't break in. (The law is different for criminal cases involving Court fines, where - subject to some safeguards - they can break in.)
4. The Ombudsman should be very helpful but - as already said - a formal written complaint must be made to the supplier first. Make sure this is sent by recorded delivery, & a copy kept.