OK I will try to make it brief but useful, your Electric Distributor (dependant on what area you live in) will provide a supply to the property. These are called the DNO. A Supplier will bill you the electric used i.e. nPower, BG, E.on etc. Each domestic property is given a 60/80/100 amp supply dependant on age and location. If you want another 100 amp supply in theory you will have to request and pay for an extra one. You will not be doing that route - trust me. What an electrition will do is install either connector blocks at your meter point and split the consumer units, or they will tap off your existing consumer unit a small dedicated ring for the loft. Whether its rated for 100amp or not realistically you will never achieve that as your house is depending on using that 100amp too.