good luck to ya then mate cos when you start looking at the implications of having a loft conversion and all the legislation that goes with it,you'll wish you never started at all,
IF you go the right way you will have to have fire routes out of the loft via a window (expensive i can tell you but cant think of the name now) when you come down the stairs you have to have an exit to the left AND the right,you have to have new stronger supporting joists in the loft wich will mean new ceilings in the bedrooms,
I had 2 quotes for my loft conversion which in the end i didnt have ,(i went and converted the garage in to another room ) both around the £20,000 mark and there wasnt too much between the quotes,
If you go your own way then you cant call it a bedroom (legaly anyway,because of the fire laws)but you can get away with just boarding up the floor and batoning up the trusses and plasterboarding the walls with insulation behind it,but beware of what the floor will carry in the way of weight, you do not want to put a wardrobe in the corner of the room and find your lower bedroom ceilings start to sag because of the weight in the loft,
I would think about this one carefully and if you dont get more quotes, i would get a builder in to at least advise you on what can be done with the minimum of cost,
hope you find this a help