Hi sparky
In my experience it takes an eternity for them to get to that stage, although it's not unknown. The debts normally pass from one collection company to another with charges being added.
Mark
Mark is not posting regularly in the Trust-deed.co.uk forum.
Whoever he owes the money to can petition for bankruptcy - that might be the debt collection agency if the debt has been formally assigned to them, or it might be the original creditor.
They would have to be owed at least £3000 first and would normally have to raise an action for decree in court first then serve a charge for payment (though there are other routes that would qualify creditors to petition too). As Mark says, it tends to take a long time, but it certainly can and does happen regularly.
More likely is that they would seek to recover the debt by means of a bank or wage arrestment before they got to the bankruptcy option.