Five point harness above 18 kilos?

[Bouncearound]

Dd is just three and weighs just over 18 kilos. We have a two way elite and maxway in our cars but need something for use in grandparents cars 2-3 times a year (they live far away so we normally fly to see them). I'm ok with a forward facing seat on these rare occasions but can't find many that are five point harness and suitable for over 18 kilos (yes, she is just slightly chunky but really tall!) I am trying to avoid a high back booster unless absolutely necessary...
Any advice would be much appreciated

I have tried out a number of car seats with 5 point harnesses suitable for children over 18kg.
I tried a Britax Advansafix which really should be done for trade descriptions as there's no way a 25kg can get anywhere close to wearing the harness! My son is dinky, and he had grown out of it just after he turned 4. Be Safe have a rear facing seat which only had 1cm of growth in it at the same time, and the retailer refused to sell me it with so little room for growth. He sold me a Britax Traveller Plus which was just horrid. Old fashioned and the straps twisted CONSTANTLY! It used to take me 10 mins (not joking) to untwist and rethread the straps before I could use it each outing.
I then got a Diono Radian 5 which weighs an absolute tonne - seriously the most heavy car seat I have ever tried! This worked fine, and my son at around 110cm tall had some room still to grow. However, I didn't like the head loll when he fell asleep (forward facing) and I didn't want to rear face him (which is possible).
I then got an Apramo One which forward faces with a harness up to 25kg too. There was similar shoulder height space to the Radian 5. It was a bucket style seat, and reclines. However, if you need a recline, ensure you fit the seat reclined otherwise there is no room to recline as we found out. A very good seat and nicely padded.
Finally, we have ended up with a Cozy 'n' Safe Excalibur. We had the last three seats at the same time, and I tried both my children in all 3 in the house. This one came out as the most comfortable, and has the highest shoulder height. It is really well padded, and has an excellent recline. My only concern is that it is a fairly unknown brand, sold by Tesco, but I don't know if that reflects on the safety of the seat. I haven't seen any independent test results for this (or indeed any of the others). I know cheap doesn't always mean poor quality, so I am reserving judgement. However, there's nothing that is making me stop using it, and it is currently in my car.
I don't believe apart from the Two Way Elite, or special needs car seats that there is anything else with a harness up to 25kg, but if anyone knows of one, I would love to know.

Thanks, that's a really helpful post. I'm still not much closer to making a decision... It's tricky because with our cars we bought what we thought was the best, safest, longest lasting and just paid what it cost but for a seat that will be used maybe 4-5 times a year it seems ludicrous to pay over £200! There are no younger siblings in the family so it won't be used after dc3, otherwise it might be an investment. There's also the issue of storage space in mils house for 45-50 weeks of the year. My search continues!

The Radian 5 does fold up - but its not simple like it appears to be. It takes a while, but may be worth looking at. It's just incredibly heavy - I can't lift it on my own for example and I am no weakling!