Hmmm OK - I think you're going to have to compromise on some detail - I don't know of any all stages car seats which definitely perform well in every mode, especially for a lower price (if that's a concern). That is a kind of car seat unicorn!
How old is the eldest and what type of seat are they in now?
You could look at BabyAuto Dupla which is probably the only all-stages car seat I think is worth a look just because it's rear facing up to 18kg, then you can use forward facing with harness 9-25kg, and use as a booster seat as well.
However, it needs top tether, if you're using the harness over 18kg. And we don't know what the safety rating is of it because it has never been through an independent test like Which? etc. If you don't need the extended harnessing or won't use it, it might not be the best choice.
So if not it might be better to get two separate seats, which you can do for about the same cost.
For example, a high backed booster seat or Britax Evolva 123 if you need/want the forward facing harness, for the eldest to use now. This performs well with the harness and as a booster seat, although it's a bit more expensive than just a booster.
Then something like Joie Tilt for the youngest to use rear facing until they are too big or get fed up of rear facing, then they can move into the Evolva (which is why you may want the harness capability).
Evolva + Tilt together would cost about £160, which is actually cheaper than the Joie Every Stage. You can sometimes find the Tilt on offer cheaper than that. Graco Extend is also very similar and has just come out. If you keep an eye on the prices of those two, you should be able to find one or the other at some point in the next 8 months for about £50/60.
If you have more budget available and want to maximise the performance of each type of seat, then I'd get a spin seat such as Joie 360 Spin or Britax Dualfix 2R as your combined rear facing / forward facing seat - they do well both ways. If your eldest is still under 18kg, get that first and use it forward facing for them.
Then at the same time or later, get a well rated booster seat. Joie Trillo LX is in the top ten and costs about £60. Or you can go slightly better and get Joie Traver for £80. If the eldest is older and already using a booster seat, you could get this first and the spin seat later.
This combo would cost about £260.