Best weedkiller to kill a lawn quickly?

[wassap]

Guys now that the weather is getting better im planning on redoing the lawn, quite frankly its rubbish, so i want to kill it all off, level it all and then whack some turf over the top!

So what can i use to kill it off, afaik, you have to be carefull as some stuff prevents growth for ages (glycosulphate or summat?)

Any advice appreciated.

 

[wassap]

I have a lot of weeds in the lawn too you see, hence the weed killer, if i slice the top off the weed'd roots will remain in the soil.

 

[wassap]

Thats not going to kill the weeds tho is it? the root will remain in the soil and come back to haunt me!