I'm in a similar position, DD2 is 3.5 yrs, just short of 18kg and about 110cm - almost too tall and heavy for her britax first class (5 point harness).
Choices are - car seats which have been sold in UK for a little while, often sold as special needs car seats, such as Britax 2way elite - or the new Britax advansafix which will forward face 5 point harness to 25kg - or the new Britax Maxway which will rear face 5 point harness to 25kg.
Rear facing is still the safest at 3 years old (and 4, 5...) but I'm not sure how I would get my DD in - she is getting too heavy for me to lift easily. Plus it would be a big step to change her back to rear facing.
5 point harness, forward facing, isn't as safe as rear facing. When they do it in motor racing they have an extra tether on their head/shoulders because 5 point harnesses actually make your head jerk more as the body can't move to absorb the forces - so it puts MORE stress on the neck than an adult seatbelt and high back booster.
High back boosters (1) only work while your child sits nicely in them - lean forward to play with something or fall asleep and drift to one side and they won't do their job and (2) Smaller children are more likely to submarine - slip partly or fully under the lapbelt, causing serious injury or coming out the car seat completely.
If anyone disagrees, do say. But that's what I've managed to gather. It's a bit of a bugger. I know that I SHOULD go for the rear facing Britax Maxway, but to be honest I'm probably going to buy the Cybex X-Fix group 2/3 high back booster. It has good reviews, and has a degree of recline (to hopefully hold DD well positioned in the seat if she falls asleep). I don't do much motorway/fast driving mind, which may be affecting my decision.
Good luck!