If you have an outdated system in need of refurbishing due to wear and tear, overload, a lack of earthing, safety issues, poor DIY modifications etc., it may not comply with today's regulations (17th Edition wiring regs., BS7671), so you may need to have a complete rewire.
Replacement consumer units or fuse board, installation of down lights, extra sockets, supply for new combi boiler, RCD modifications, additional supply to garage/outbuilding...