The postal services in this country had already began the road to privatization long before Cameron came along. Became a PLC during Blair's first term, had a bizarre name change for a while, was heavily downsized (correctly) throughout his tenure as PM and was opened up to competition later on. It was Vince Cable under the coalition who finally privatised it, although Labour had proposed this in the past. It was hardly a controversial move.
For all the things to criticise the govt (past and present) for, this is not one of them. It was hardly worth keeping in govt ownership, and EU competition laws made it tricky. BT had similar problems. The postal service is hardly an investment for the present, let alone the future. Let some private shareholders lose money on it instead of the public.