But sometimes they are only acting as debt collectors which limits what they can do. They will still tell you all about their powers as bailiffs even when it's not applicable.
If they are acting as bailiffs and don't have a court order, do not let them in the house as they can go round doing an inventory of anything of value and you have to let them in again on subsequent visits. Do not leave windows or other doors unlocked.
If they are debt collectors they have no power to enter your premises.