Hi, welcome to RO. I'm guessing you are in the UK from the science selective pellets. It's not one we have here in the US, but we do have Oxbow and it is one of the best brands of rabbit food here. Having prebiotics isn't a necessary thing for rabbits. It may be in some rabbit food because they believe it might be helpful to rabbits, but it's not a common thing to find as an ingredient in most rabbit pellets. But the Oxbow pellets have all the vitamins and minerals that your rabbit needs to stay healthy. Pellet amounts really depend on each rabbits metabolism. Generally you start out with the amount either recommended on the bag or with the basic recommendation of 1/4-1/2 cup per 6 lbs. of body weight. Then depending on how the rabbit does on that amount, you may have to adjust up or down. If they are getting chunky then decrease the amount and add more veggies, or if they aren't staying at a good healthy weight and looking a little thin, then increase the pellet amount. Along with that you want them eating an amount of grass hay that is about the size of their body, each day.
You can also introduce veggies into your rabbits diet, to supplement it's nutrition. It's often best to wait til a rabbit is at least 12 weeks before introducing them, though some people wait til their rabbit is older. Introduction is best done starting with one small piece of a veggie and gradually increasing the amount if you don't see any soft poop or tummy upset. Veggies aren't a necessity if feeding your rabbit a normal amount of pellets, but are if you are feeding reduced pellet amounts for any reason, as they need the added nutrition. But even then, veggies can be a good addition to your rabbits diet. They are a good healthy treat, help with tooth wear, and are a helpful food for your rabbit to already be used to if it ever gets sick as veggies will sometimes be the only food they will eat on their own when feeling unwell. You'll find several different veggie lists on the internet. For my own rabbits, I tend to avoid any cruciferous veggies, as they seem to cause digestive and gas problems with a lot of rabbits. But some people feed them to their rabbits without any problems at all. Good basic ones to start out with are dark leafy lettuce(not iceberg), cilantro, and parsley. Some people don't think veggies are good for rabbits, and it is true that they can cause some problems for rabbits that are sensitive to a particular veggie or if veggies aren't introduced correctly and given too much too soon. But if introduced correctly, most rabbits do well having daily veggies. Barbi's site does have some good information on some things, but some of her information is outdated, as far as current rabbit health and research is concerned, including the info on wry neck. If looking for health related info on rabbits I suggest looking for your info from more medically based sites on rabbits, as well as from your vet. A good rabbit vet will have the most current info on rabbit health. I've found the medirabbit site to be the most helpful and accurate, for things pertaining to rabbit health.
http://www.medirabbit.com/EN/index_en.htm
http://www.3bunnies.org/feeding.htm#top
http://en.wikipedia.org/wiki/Cruciferous_vegetables
Tooth problems in rabbits can have to do with diet and genetics. Some breeds are more susceptible to tooth problems. Hay and veggies will help with the tooth wear. Natural forage can help as well. Things like apple and willow branches, and other rabbit safe branches, may help a lot in keeping those molars worn down. Just make sure they are free of pesticides. Lots of good coarse fiber will help keep those back teeth worn down, but even then, there may still be a few rabbits that will always have tooth problems, but with lots of fiber in the diet, you may be able to stretch the time needed between dentals.