So far as I know, warrants apply to criminal matters. Debt is a civil matter.
Debt collectors (as opposed to bailiffs) have no legal status under the law, and you can deny them access. If there is a County Court Judgement (CCJ) for the debt then the creditor can - with Court authority - instruct a bailiff to come if the terms of the CCJ are not complied with. Bailiffs dealing with civil matters are not allowed to enter a property by force on their first visit, but can enter if they can get in through an open or unlocked door or window.
often get in by trickery , so watch out for the lies at the door. Best to never open a door, or window , unless you know who is there.
The plod will normally attend if there is a warrent to execute granted by the court , to see no breach of the peace is caused.
They would need a warrant. They can obtain this only if they are bailiffs I believe. I would contact them and seek a repayment plan before they come back.