You aren't butting in!
There are no chest clips in the UK because standards are different. Your seats are fine without them but the seats here require them
For forward facing, if the straps are below the shoulders, it can push down on the child and compress their spine in a crash. For rear facing, if the straps are above the shoulders, the child can "ramp-up" in the seat in a crash and we want them to stay put so there is less force on their spine. Does that make sense? I'm not familiar with seats in the UK though so check the car seat manual on strap placement. But yes, we mean where the harness goes into the back of the car seat by their shoulders. Check your manual on how to change the slot height.
You are more than welcome to post pics too so we can take a look.