Standard car seats only have 5 point harness to 18 kg (39.6 lbs - 2 stone 12lbs). Even group 2/3 car seats require that when the child reaches 18kg the 5 point harness is removed and an adult belt used.
We've just bought a two-way elite for DP's similar aged DD, she's also tall for her age. She hasn't had a chance to try it out yet but the women at Essex CC's car seat Try before you buy thing reckon it can be used for children up to 5 or 6 and although it's rear facing, it can be turned round to be front facing. In relation to the height issue, they said that the key thing is the child's back length (and this is a seat suitable for a tall child) rather than leg length.
The fact that it can be used for (hopefully) a few years and is rear facing were the clinchers for us when making the decision.
It might be worth ringing the incarsafety place in Milton Keynes for info as they do seem to be really clued up. Also I don't know what help you can get from Essex if you aren't local but it might be worth seeing if they can give some advice over the phone.