Is your 2yo rear facing or forward facing?
If she is rear facing and you'd like to carry that on, I would look at seats like Britax Max Way, Axkid Move, Minikid. These can be used up to 25kg which is approx 7 years old so plenty of time yet and after that you can pass it down to the baby and move your eldest into a booster seat.
If she's already forward facing or you want to change to forward facing now, I would look at the 123 type seats which start out with a 5 point harness and then you can take it out later and just use the seatbelt. If she's fairly average/small for age, then one with an 18kg limit on the harness would be fine - I like Britax Advansafix, but Recaro Tian Elite also got a good crash test score. Although Maxi Cosi seats are good in general, I wasn't a big fan of the Titan 123 seat because the harness on it is very small and wouldn't fit most children until a safe booster age. If she's really tiny for age then it might be OK. All of these need the top tether strap as well as the isofix. If you're going for a 123 seat it's worth getting one with top tether as the ones without don't perform as well.
If she's bigger for her age or quite tall or you just want to use the harness for longer, there are a couple where you can use the 5 point harness for longer - Joie Bold, BabyAuto Dupla and Cozy n Safe Excalibur. These use top tether as well.
I would not look at the 3-12 year seats yet as these tend to be the booster type which you use with the adult seatbelt, and at not quite 3 she's probably too light (min 15kg) to use these seats and it's likely she's not mature enough to sit sensibly with the seatbelt either, most 3 year olds aren't. You ideally want to be well past the tantrum-and-nap years before you switch to a seatbelt seat!
I know it seems like spending out loads of money - if it helps to know, this is quite a front-loaded expense, as the toddler (5 point harness, sitting up) type seats are the most expensive and complicated. In recent years, the infant (reclined, carry handle) seats have become very expensive too with the bases and the different options, and I think they are a bit overmarketed, because when you're pregnant for the first time you don't tend to know anything about car seats and they have you then. But not to worry, as you've had really good, safe seats and you'll probably only need to buy one more in the harnessed stage, once you get to the high backed booster stage, they get much cheaper (and don't need bases!)
All the options I've mentioned are around £200 though - if you wanted a cheaper option you could look at (Around £120-150):
- Britax Evolva 123 - this is a belt fitted 123 seat, harness until 18kg then booster.
- Maxi Cosi Tobifix - this is just a Group 1 seat up to 18kg with harness. You'd then need to buy a separate booster after it's outgrown. Forward facing.
- Graco Turn2Me - a spinning seat, rear facing and forward facing up to 18kg. Has a slightly more generous height allowance. Could use for baby to take advantage of the spin, and keep her in her current seat.
Or really budget options (around £100):
- Britax First Class Plus / Maxi Cosi Priori, if you just want a basic Group 1 forward facing seat
- Joie Tilt or Joie Steadi, (or even Joie Juva - lasts about a year longer than Maxi Cosi Pebble) if you want an interim rear facing seat for baby, to allow DD1 12-18 or so months more in her seat before moving to a booster.
- Joie Stages, if you want a cheap rear facing seat for DD1.