I'm a member of Genes Reunited, although have never bought any credits there. If you put the name of your ancestor in the search box (section 3 of the first page), you can get a free census index, but you have to buy credits to get into the actual census entry showing other household members. The more info you put in the search, the more specific will be the info coming up.
In your question, you are talking about the BMD entries. This is the same as you can see in the indexes at the National Archives. Again I haven't done it through Genes Reunited, but I have Ancestry UK. There are 4 indexes per year, one for each quarter. I would guess, although I don't know, that you would have to use a credit for each one you look at, which could make it expensive if you don't know dates. On Ancestry, you pay a flat rate for the month/year, and use is unlimited. You have to look at each index. It gives you a reference number you can use to order a certificate at the online lesser rate of £7. The index entry you get on screen is not detailed, it only shows you the quarter of the year the event was registered, and the area. For marriages, you get a page up showing eveveryone married for that quarter in the area. You have to do some detective work to find out who married who, and you might end up with a lot of unwanted certificates.
I find it all very addictive, and once you start, you won't want to stop. Unfortunately you often come up with more questions than you can answer.