I think the 18Kg limit is just for the rear-facing child seat itself not the LATCH anchor points. If the car seat limit for forward facing is 40Kg I don't think there is any problem with the anchors holding that.
When I put my kids' child seats in the car, I tighten them down hard and they don't give, they're bolted to the car and I don't think your child and seat is enough to rip them out.
edit- as I read a little about them it kind of looks like the limit is 40Lbs. which I think is 88Kg?
here's a quote "There is growing interest in using the top tether and lower anchors for children in harnessed forward-facing carseats that have weight limits above 40 pounds. The government safety standards do have a safety margin above that, and were based on a
combined child and carseat weight of 65 pounds. It is possible that some automakers added a safety margin above that. Even so, it is best to check with your vehicle's manufacturer and carseat manufacturer if this is an issue, especially if your child exceeds 48 pounds. Unfortunately, some companies may not have this information readily available for consumers."
from
this page
I would say "drive more, worry less"