The Office of Fair Trading disagrees.
As above, this isn't how it's meant to work. You don't legally get to harass people for a while, then start a legal action, then outsource debt collection if the court doesn't issue a judgement / wage garnishment order.
I'm not taking any position as to whether or not this is a good idea - I'm just pointing out that you need a license to undertake the described activities in relation to a debt for which you aren't the creditor (and you don't meet one of the 2006 Act exceptions). Otherwise you face investigation by the Office of Fair Trading, penalties under the Consumer Credit Acts, and possible harassment charges under the Protection from Harassment Act 1997 depending on your specific conduct.
There's the strong possibility that your mark isn't going to know about the legal rights available to him/her, or might be all kinds of dodgy as far as the original loan, tax avoidance and other matters. In that case, you might be able to get away with doing whatever you want to do without having the full weight of the law behind you. But there is very little room for debate as to whether the law currently requires you to have a licence. What you're proposing falls very clearly under the requirements listed for licencing. if you're doing debt collection prior to initiation of a court case and you aren't the creditor, you need a licence.
What's more, the lending companies or individuals themselves may need to be licenced, particularly if the loans are denominated in £ rather than BTC (and for recovery purposes they would likely need to be). It's debatable what the court's position on BTC-denominated debts will even be, but the Consumer Credit Acts set out licencing and reporting requirements for business engaged in the business of lending. So if you got yourself into trouble in this process you could also get your clients into trouble for simply being in the business of lending. More info here.
This could eventually work, and it might eventually be called for. But unless you know something big that I do not, you may have to substantially adjust your proposed business model. And my guess is that at the moment there isn't a large enough UK-only BTC debt market to support it. Then again, if your lawyers believe that you can get away with doing it, go to town. But if you're doing this in the absence of good legal advice, I'd strongly suggest you seek some before you take any further steps to make this happen.