Check water tank in roof for overflow outlet

I need a plumber to go up into the loft and check that my water tanks have an overflow outlet (because I cannot find it on the outside of the property) and also is there an expansion pipe in case the hot water system fails. In addition, I have very low water pressure from the taps throughout the house, particularly in the bathroom. Can you advise whether it's possible to have a pump fitted to the tank that serves the whole house? Finally, I'd like to see if it's possible to move the main stop valve for the house under my kitchen so that it is more easily accessible.