Why is it that you can see some marriage certificates, no problem, on Ancestry, but others you can't.
The general register office started recording births, marriages and deaths in 1837 and this would be the start of marriages in a register office along with churches. This is government records.
So if you got married in a church then the vicar every quarter gave the other certificates to the local register office and they kept a copy and sent on another copy to the general register office that was, at the time in London at Somerset house (now in Southport)
The Mormons came to UK during the 1930s and 50s and they photographed all the parish records they could, some vicars were obliging and others weren't. They did this for their own religious reasons. They then set up family search online and have many records transcribed to search. The films they made have been used by ancestry and uploaded digital and thus only some marriages are available online and others aren't. Some archives have done deals with ancestry so may have also let the company use their films of marriages from parish registers.
So this is why the records on ancestry are not complete and some baptism and marriages will be on their site and others are missing
When you look at the records with just the names of the bride and groom - this is the General Register Office Index - so you can order the copy certificate from the GRO 0300 123 1837 is their telephone number to order and pay - you pay for the admin cost of searching the millions of certificates and making a copy.