The data protection act does not state that a company in the UK has to give a reference based on opinion. The organization you worked for was not breaking the law by giving fuller references which may have included opinion but rather opting to base their HR policy and best practices on more than just the data protection act (this is the case for the majority of large organizations in the UK). For what it's worth i'd say that the organization you worked for got their policy right and that 99.9% of the time employees would have little course for concern over the accuracy of their references. Unfortunately in the increasingly litigious world we live in, businesses seem more and more inclined to focus on that tiny minority that could cause them a big and expensive problem.

While you may not have experienced a defamation law suit that came about as a result of a reference that does not mean that they don't exist. Searches of legal forums such as the Law Donut would provide you with specific cases.