"I'm struggling to find English birth/death/marriage certificates online - is there a certain website that holds them all?"
As others have said, the GRO is the place to go (you will need to make a free account with them):
https://www.gro.gov.uk/gro/content/certificates/login.asp
As @ProfessorBinturong said, there are three levels at different prices. But it is only birth and death certificates (births between 1837-1934 and deaths between 1837-1957) that you can get a digital scan. For marriage certificates you have to pay the extra for either a pdf or paper copy.
As she said about digital scans, "If the one they send is bad you can claim a refund but I don't think they do a rescan. You can then only get a PDF or printed one." This happened to me once, I got a refund and was told that they cannot be rescanned.
A paper certificate is £12.50, a PDF (if it is available) is £8 and a digital image is £3.
When it comes to searching for births or deaths (not necessarily looking for a certificate, but just wanting to confirm a birth or death) then I've found the GRO search function to be very useful as, for example, when it comes to births it will show the mother's maiden name where that is different from the birth surname.
https://www.gro.gov.uk/gro/content/certificates/indexessearch.asp
But if you are looking for a marriage certificate then you do need the GRO Index Reference (although you can pay £16 to get somebody at the GRO to search for you)
The GRO Index is what is shown on Ancestry from the England & Wales Civil Registration Marriage Index (up until 1915 it doesn't link the names of those marrying, but from 1916 it will also give the surname of the person that they married), when you do a search for a marriage.
A typical example that you will see on Ancestry might be:
Name Walter Loynd
Registration Year 1881
Registration Quarter Jul-Aug-Sep
Registration District Bolton
Volume 8c
Page 484
https://www.ancestry.co.uk/search/collections/8913/records/18315955
This is the information you need to supply to the GRO to request a marriage certificate from them.