What is the best way to kill off my lawn so that I can get rid of the weeds and dig out the excess soil?

sorry folks covering it with anything wont achieve a great deal as once uncovered the weeds will be back, a lot of weeds like thistle, nettle and docks take more than one dose of killer and while your doing that new weed seeds have arrived, rotivate remove excess soil and seed or turf, once re-established use selective weed killers, like moss or broad leaf weed killer that will not effect the gr at the same time use a gr fertiliser all to manufacturers reccomendations. not forgetting that when you buy turf or seed there is a multitude to buy from. ie hard wearing, (no i`m not joking). all else failing pop into your local golf course and ask one of the groundsmen you usually find that gardeners are keen to share their knowledge