Oh dear, seems to be some confusion here. Do not, under any circumstances, use SBK in such a manner - it is a brushwood killer, intended for use on tree stumps and woody bases to plants - spilt or applied to soil, it will make the soil toxic for up to six months and possibly longer.
If you are able to water afterwards, the thing to use on lawns is a specialist Lawn Weed, Feed and Mosskiller treatment, but it must be watered in within 2 or 3 days, either by plenty of rain or by you with the hosepipe. As we are now in drought, I wouldn't think about scarifiying until the Autumn. If you have large patches with no grass and just weeds, then its worth digging those areas over, removing the weed roots by hand, levelling off and sowing grass seed, which again must be kept watered. You must decide which is the best approach - once you've used a chemical product like Lawn Feed and Weed, seed cannot be sown for six weeks.

I had thought as much but the reason i even thought about using SBK is because on the bottle it say 'kills weeds not grass' which to me, intimates it can be used on grass. Thanks bamboo i will put the SBK aside and get in something specialized for lawns.

The lawn is more than 2half acres, so i am not sure how feasible it would be to hand weed in the amount of time i have to work on it. The scarifier was bought specifically for the lawn as they insist they want to scarify it in the next couple of weeks, this is a struggle i face - Do you do what the 'customer' asks even if you know otherwise,that it is not the right time or not the right thing to do and have explained this to them, yet they want it done anyway?

Ah, well there's a dilemma that only you can decide - I've been gardening for people for 30 years, and have never yet done what the client insisted on if I didn't agree with it, even to the extent of refusing to take on a job because the owner was intransigent and only wanted certain things done, when the whole place was a neglected wilderness. I told him to find someone else, I'd either do it properly or not at all. He called me 4 weeks later and I worked for him for 6 years, till he moved... but he might not have done, he could have got some other obedient gardener instead of me.