@Tfoot75
My 7yo is only just over 25kg - she's a few cm from being tall enough not to need a car seat at all. (she's ff since about 12 months as was the norm at that time)
Another one here who doesn't know anyone who rf after about 18 months.
OP I believe it's a mostly theoretical risk - I'm not aware that there have been any cases of internal decapitation (which is what a rf seat protects against) in this country while in a legal car seat. It is a crash test scenario that in reality has a 1 in several million chance of happening.
It's not a theoretical risk, sorry. These kinds of things aren't publicised because of course nobody in their right mind is going to say to a parent who has just lost a child "Well, it's because they were in a forward facing seat" (also you simply couldn't know that based on the information you have at the scene of an accident) but there have been massive scale studies done across Europe, across the US for example, which show a clear difference in outcomes. There is also a lab in Sweden where they recreate accident scenarios where children have died despite being in car seats and they simulate the crash using computer systems (I believe) and generally they find that in most cases a rear facing seat would have saved the child's life. I wish I could find the article I have saved about this but I have too many car seat links bookmarked now so I can't

. I must stress, we are talking about an absolutely tiny number of accidents. The vast majority of children are protected well in modern car seats whether they are rear facing or forward facing, but there are a certain unlucky few and this is absolutely real, not theory.
In this country - I don't know specifically. But physics doesn't change because you're in a different country and UK uses an international (UN) regulation to approve car seats.
OTOH, there seem to be two different goals and I think people misunderstand this - there is the goal of reducing fatalities/serious injuries as far as humanly possible, in which case rear face for as long as you can or to a minimum of 3 years 9 months, or there is the goal of reducing fatalities/serious injuries in general in which case you start with the most vulnerable group - the youngest, and those improperly secured - and work from there.
In real life we do not always follow best practice, it's a case of balancing risks against another. If you want to be very pedantic, taking children in a car at all is a risk that we do not have to take, but most people do choose to take this risk because it is practical. Therefore we use car seats to mitigate (even though not entirely eliminate) the risk. You could take the same attitude to forward facing when practical/preferred. Accept that it is a higher risk and make sure to do things like always tightening the harness correctly and ensuring not to overload the limits of the seat.
There is sometimes a feeling in pro-ERF spaces that it is possible (and therefore preferable; therefore "good parenting") to eliminate ALL risk from a child's life and I do not think this is helpful or practical. Also I see parents in utter turmoil and anxiety when it comes time to turn their children around sometimes at 5 or 6 years old! Anxiety about a six year old in a booster seat (unless they have special needs) is misplaced, and says to me that the message isn't quite getting through with the correct level of nuance.