Who are BPO? They're not a name I've heard mentioned in relation to private parking charges.
It would also be somewhat unusual to hear from a debt collector after the matter has escalated to solicitors. Are you sure this text from BPO is in relation to this parking charge?
Moorside won't have sold the debt as it is not theirs to sell, Alliance is the (alleged) creditor, not them. In practice, we don't see parking companies selling debts - they engage debt collectors, but remain the creditor themselves (from memory, their KADOE contract with DVLA prohibits them from selling any alleged debts, but don't quote me on that).