you can sit to the trot at prelim but some judges will not like it as it can restrict the movement of the horses back if not done lightly enough. (so i have been told anyway, i am no expert!)
he goes alot better for me (and i ride better) in sitting, when i rise it feels slow and then i rush him, as for he comes down onto the bit when i am in sitting, also is there a perticular bit you have to have or can you have most things?
You have to have a snaffle, but can be a variety of snaffles, eg. hanging check, french link, loose ring, eggbutt. Not very good with dressage, someone else may be able to help more with which bit you are allowed
ETA: Found the rules on the BD website, might help with the bits, you have to scroll down to 4th or 5th page
Lots of people who sit at prelim affect the horses way of going negatively, they feel it is better, but in fact they slow the horse, the horses sometimes stiffen over their backs and so do not step through and the whole picture is worse than if they had risen. In most instances at that level horses are free-er, more swinging and more forward when the rider rises. However, if you feel (and your instructor feels) that you can positively affect the horses way of going by sitting then you are allowed to.