As far as I know, a person has never needed a degree in law in order to qualify as a solicitor or barrister. You don't need a PGDL in order to do the SQE, but some law firms currently prefer non-law graduate applicants to do a PGDL before doing the SQE.
I've never heard of the Diploma you mention. At 900 quid it sounds like it might be a bit bargain basement. One of the problems with the law (and indeed life in general) is that you get what you pay for. Buy cheap, pay twice is often a sound maxim.
In the law, you can have a cheap lawyer, or a good lawyer. Choose one of the two. Is the same true of law schools? In that context, expensive does not always mean good. I think that many of the law schools selling postgraduate legal qualifications charge a lot for not much. But the market allows them to do that.
What does the Diploma you mention cover? I do not suppose that, at 900 quid, it will cover each of the core subjects covered by a PGDL in much detail.
A PGDL is the bones of a three year law degree taken in one year. It provides a foundation for the development of legal skills. I have a non-law degree and a GDL. Decades later, I still rely on what I learned while obtaining that qualification. I confess that I sometimes felt a bit legally under-educated when I first started to practise law (I caught up later). I suggest that you check carefully how much of a grounding in the law the Diploma you mention may provide.