Some of your criteria are incompatible, mainly the up to 12 years one - you can get decent forward facing harness + booster combos, but they don't rear face. Or you can get rear facing seats that convert to boosters, but they don't have good safety ratings.
Since you can get a decent but basic booster for about £30-40 later on, and standalone high backed boosters are so much more practical (less heavy/bulky) I'd scrap that particular requirement and you'll find it much easier to choose :)
At 18 months forward facing is still quite risky. It would be better to wait until 2 years old, even for an occasional use seat, and better 3 years really for full time.
So you have three size categories you could move into of rear facing seats now.
Rear facing up to 13kg and then forward facing up to 18kg:
These will last an average sized child rear facing until about 2.5 years but can be a good compromise if you're definitely sure you will forward face before the booster stage, as they tend to perform better forward facing than seats which are purely optimised for rear facing. Just avoid the generic ones.
Britax First Class Plus - not too expensive, versatile fit so can be decent for a spare seat.
Maxi Cosi Axissfix - great for a main seat, especially if you get the version with airbags (!) expensive though it is. Recline is not great in rear facing. Spins to the side.
Maxi Cosi Milofix
Maxi Cosi Beryl - unusual in that it can be fitted using seatbelt or isofix and also can harness past 18kg, but the height in it is not great, so you probably won't get all the way to 25kg in it. But it's good if you think your child will hit 18kg just 6-12 months before you're happy for them to be in a booster seat.
Up to 18kg / 105cm - this will last an average sized child until about age 4. Most of them have the option to rear face and forward face, many of them spin to the side to help get the child in. The belt fitted ones are fairly lightweight and inexpensive so can be really good for your lesser-used seat - good options for this are Joie Tilt/Steadi/Stages or Graco Enhance. However, I wouldn't recommend using these seats forward facing at all. They are just not optimised for it and have no safety features to reduce the extra risks of forward facing. They can be annoying to use as a main seat as in rear facing, the seatbelt goes across the side of the seat (like the infant carrier when belt fitted).
If you think you will definitely use it forward facing then I'd go for an isofix one as these tend to have a support leg to help reduce forward movement. Good options (in both RF/FF mode) are Britax Dualfix, Joie 360 Spin, Cybex Sirona, Besafe izi Turn.
There is one RF seat up to 18kg which can use the harness up to 25kg FF - BabyAuto Dupla. But there is no independent safety rating on it. Can be a good option if you know you'll definitely need/want to harness up to 25kg, and need to rear face for now, but don't want to rear face all the way up to 25kg. Might be an option for your second/spare seat although needs top tether.
Rear facing up to 25kg / 125cm ("Swedish seats") - these are the granddaddy of rear facing seats and work incredibly well. Last rear facing for an average sized child until age 6/7. For larger than average kids you'll need the 25kg capacity to get to age 4/5.
Tend to be lighter than the 18kg isofix ones, less expensive, take up less space in the car while giving more space for the child. The downside is that they are only rear facing and need to use extra tethers to fit which can be annoying in a second car (though if you'll use the same car all the time you can buy a second set of tethers and leave them there). Most of these have passed the Swedish Plus Test which is the strictest car seat test in the world.
Examples:
Britax Multi-Tech 3 (Actually, this one can forward face up to 18kg harnessed as well, but it's the only one that can)
Britax Max Way / Max Way Plus
Axkid Move / Minikid
Sorry that was really long but hopefully it gives you a few starting points?