Brambles in the turf

No instant result, simply keep cutting them back and they'll eventually (over several years) give up.

failing that you've got to dig their roots out (advisable as they tend to creep & spread regardless, EVEN dropped cuttings root SO easily if left!

ULTIMATELY. if you've got brambles, deep root killer then leave the ground 6 months before turfing (ie nov / march) just popping in whilst its bare in order to dig some roots up while you wait.

If you try to shortcut (ie garden make-over show speed, or a few weekends bish bosh speed , then you're wasting your time & will be plagued later.

did the original poster solve their problem?

please thermo dont try and advise people about Japanese knotweed!