you do need a dish installed outside the house, it comes part of the package.
if you want to watch different channels in different rooms you will need to go for multiview, which is 3 seperate boxes (each one cabled from the dish)
otherwise you can either split 3 cables from the one box, or get digisenders(wireless tv signal trransmitters which are available from argos)
if you go this route you can only watch the same program in all rooms.
you can alos not tape from a different channel than the one you are watching (unless you go for SKY+) which requires 2 cables from the dish.
if you go for multi room, or SKY + then you need all boxes connected to the same phone line.
If you go for any multi room you need a box in each room.
not sure if it quicker to go through local guy or direct to sky, they all have to get their cards from sky, you could try and book an appointment through sky and the local guys and see which can get to you quicker.