The OP is talking about a CCJ. The only way of getting a bailiff to act for a CCJ is through the Court. The creditor cannot do it him/herself.
If a firm such as Marstons – who do act as bailiffs - are appointed by the creditor they are acting only as debt collectors – not bailiffs. They do not have the same powers as bailiffs and, if the debtor is aware of their lack of powers, they can simply refuse to have anything to do with them. On occasion their results may be got by threatening and sometimes illegal behaviour, (although this can also occur when they are appointed as bailiffs). I’ve no objection to “firm & persistent” but would draw the line at over-aggressive behaviour aimed at misleading and frightening the debtor about what they have the power to do. For example, if they are acting only as debt collectors there are no circumstances in which they would have the power to enter the debtor’s property or to clamp & take effective possession of the debtor’s vehicle.