No not every debtor is vulnerable, but remember that allowing charges to spiral many multiples of the original debt as can happen now is also unhelpful. I would suggest that no more than £50 and a maximum of 25 miles at 40 pence per mile for private individuals, is the ballpark.
Obviously if it is a corporation and they have funds, then £300 plus mileage, and legitimate true cost of removal vehicle, remember that a Luton 3.5 tonne vehicle is around £75 per day to hire if self drive, plus £8 per hour for a couple of porters, the hourly rate to cover duration and attendance plus two hours for base job and return to base, bear in mind that loading these fees upfront all together is naughty and should only be imposed if HCEO calls subsequently specifically to remove.
As to sticking to the point, not all debt is wilful avoidance, and is due to a change of circumstances severely reducing income and ability to pay, that is my point, that any extra makes successful timely discharge of the debt less likely. This is true whatever the cost of enforcement.
In modern society with electronic systems, affordable deductions from salary/benefits is an equitable alternative with minimal cost to the creditor if systems were put in place.
Sorry if I cannot see the value of your work, but distress is medieval and deserves no part of the system in the 21st century. As an advice worker and Councillor I have seen the results of excessive enforcement demands, and action by Bailiffs and HCEO, and they could well amount to breach of Article 3 European Convention on Human Rights, as cruel and inhuman treatment that could be construed as mental torture.
There is one underlying issue that is the elephant in the room, you run the risk with excessive enforcement charges coupled with the existing debt, sending the debtor into bankruptcy, at which point it is effectively game over for enforcer and creditor, who may then get diddly squat.