... its not possible to buy them all. I suspect that is the case for most researchers.
My time is free though and, even if it takes a long while to find an alternate source for the info, for me its worthwhile. ...
That's perfectly sensible. I certainly don't buy all the certificates for BMDs in my tree - though I buy more for Scotland because they're so much cheaper. In England & Wales, I usually only buy certificates for my (direct) ancestors - basically because I got paranoid about what a cousin had done - she had made the mistake of believing a census about our mutual great-grandfather. She therefore had him, with his supposed parents, in Preston when he actually came from Penwortham, a village across the river from Preston. By good fortune - not deliberately at that time - I'd got all his censuses, the earlier ones of which gave the correct birthplace. After that, I made sure I got everything that was
feasible. (And everyone's definition of feasibility will differ).
The only thing that I'd encourage people to think about is the idea that their time is free. It isn't. We may pay for it in different ways but it's time spent that we'll never get back again, time that we can't spend on something else. It can even lead, indirectly, into cash - far better for me to spend one full day at Chester Record Office than two half days - that's one lunch rather than two! And it might be that spending cash on a certificate enables me to short circuit the process and do one full day now rather than a half-day now and another in the future. (Depending on what I pay for lunch and transport it might even be cheaper!)
Certainly, in this case, first having a look for any non-conformist marriages online would make excellent sense - that's why I mentioned the possibility. The only thing then is how long do you spend looking? As suggested, LancsOPC is definitely a place to look (you may have done so already) and so are FindMyPast (for Cheshire records) and Ancestry (for Lancashire records).