As has been said, you can't get the certificates. Each has to be ordered, individually, at a cost of £9.25 - so you really need to be double/treble SURE you've got the right one before ordering it.
Every county/parish/town/place has different and differing coverage. You really need to "get to know a specific parish inside out" if it turns out your people settled in one spot.... but, as a rule of thumb I'd check:
► freebmd.org.uk - can I find any evidence they lived/married/died? All you get there is the quarter in which an event occurred (1837 to about late 1970s).
► freereg.co.uk - parish record transcriptions, where you might be lucky enough to get some "extra notes" made by the Vicar including father's names, occupations, age, where the bride/groom came from - but it's hit/miss which parishes have been transcribed and how good the Vicar was at filling in all that juicy extra info.
► findmypast.co.uk has the best English parish coverage - they're now working with local Family History Societies by providing the FHS transcriptions, but they're not all there and/or not all there YET.
► Ancestry.co.uk has its uses... you can get a clue where you could look next with a free membership and just seeing what searches throw up.
►
www.gov.uk/search-will-probate has wills/probate indexes searchable online, although a bit tiresome (unless you're clicking through from an ancestry/similar search result) they can yield an address or a next of kin/wife's name.
Beyond that, each county has an archives depts - and some counties have made available their BMD indexes online, providing more complete coverage than freeBMD, but no extra information usually. Some archives have an online searchable catalogue that can also yield the occasional snippet that fills in a gap or gives you a clue.
There are some OPC projects; volunteers in a few counties that have transcribed anything they could find. e.g. Lancashire, Cornwall, Devon ....and a few others.
It's really hit and miss and, for each person, you really need to try all of the above sources to try to piece together enough information/evidence that a person existed, where, when and what you want to find out next.