Hi Terpe,
Like everyone else I am sorry to hear you are having this problem.
RE the care of address. I don't see that whether a person puts c/o or not an address alters the fact that your address was a forwarding address. Legitimate business use forwarding addressing, maybe a company based outside UK would do have a UK forwarding address. They would probably never tell their customers that was the case. I believe, although I cannot find anything on the internet, that forwarding address agencies do have certain legal responsibilities/liabilities, including keeping a register of the person using the facility, and their proof in ID etc. but in you case that cannot apply.
If the guys from the debt collection agency calls again I would tell them to leave and if they show any signs of abuse/aggression I would call the police and say you fear for your safety. I doubt that police would actually do much but your call should be a logged message.
If a person is on or in your property (as a trespasser and not doing anything but refusing to leave) you can call upon the police to help you remove them. However the police when doing that are acting as private individuals not as police officers, as trespass is not a criminal offence. If they are being abusive or threatening the police can arrest.
There must be many people who try to evade their debts by using a friends or a false address, and maybe in many cases the person who lives there may know the whereabouts of the person, so its not surprising that debt collectors probably 'try to put the frighteners on' in the first instance. But as you have now done all you can to prove that you are an innocent victim they should back off.
If they continue to contact you, you could try a registered letter alledging threatening behaviour, to the Chief Constable of the police service who cover your house, with copies to say the CAB and your local MP. From experience, I know that letters which have been copied to other eminent people trend not to 'go astray' in police systems.
