Just for info, the reason some people gave you incorrect information about it needing to be taken sequentially is because that was true until very recently. "Additional Paternity Leave" I think was the name of the old benefit. The new benefit came in last year, I think.
The easiest way to think about it is to think about your statutory maternity leave - you can have up to 52 weeks off, but only 39 of them will be paid, right?
So split that into two components - unpaid leave (52 weeks) and paid leave (39 weeks). What Shared Parental Leave lets you do is give "binding notice" to return to work, and then turn any unclaimed weeks of unpaid leave into a pot of leave that either you, or your husband, can take at any point during the child's first year. You can take it in up to four blocks (though you have to give quite a bit of notice).
Shared Parental Pay is the same thing, except it deals with the paid leave aspect.
So, let's say you give your work binding notice to return after 34 weeks. You can then create a pot of 5 weeks of paid leave and 18 weeks of unpaid leave. You can use this between the two of you as you see fit across the child's first year. Perhaps your husband will take 10 weeks off, with 5 of those being paid right after your baby's birth. He could then also take another 8 weeks unpaid after you go back to work to ease the transition.
Does that make sense?
As to eligibility, to take either Shared Parental Pay or Shared Parental Leave, both parents have to meet an earnings threshold. For the unpaid leave, it's really, really low. It's something like having earned at least £30 per week over the last year, and I don't think you even need to be earning that consistently. I think you only have to have earned that over 13 of those weeks.
Shared Parental Pay is a higher threshold. You need to be earning an average of £112 (I think? It's close to this number) per week to be eligible and they work that out based on average earnings over an 8 week period that ends about four months before your due date. Again, apologies I don't recall the exact specifics, so if you are borderline for earning that much, you'll want to check the government website.
If you DO earn that much, then Shared Parental Pay is paid at the same rate as statutory maternity pay - i.e. about £140 or 90% of average earnings, whichever is lowest.
Hope that helps.