I used old spec papers preparing for A-level physics, because there weren't many new spec papers and it is a bad idea to waste them early on. (I started using them about a month or so before the final exam, because there were only about 5 sets of them) Whereas you have plenty of old spec papers, though probably fewer than I did because I was on Edexcel which has IAL papers that are very similar to the home papers.
They were pretty good, and formed a huge chunk of my revision. I tended to focus on papers with low grade boundaries and avoided those with high boundaries, typically because the former tended to have quite formulaic questions that I had already practically done before in previous years. I did try to do as many as possible though, and ones that I didn't end up "doing", I flicked through in the weeks leading up to the exam.
Do note that there are some topics that are not covered my the new spec and some new spec topics that are not covered by the old spec. I found that the latter scenario was much more common than the former. For that purpose I used papers from other exam boards that did have those topics on the old specification.