You have way more choice than that, just in case you weren't aware - generally the lists on the buggy pages are incomplete because new car seats come out all the time, plus they don't really want to advertise that there is loads of competition to their own!
You can get Maxi Cosi or Britax adapters for the Uppababy.
The Britax ones just take Britax seats, so the Babysafe or the Babysafe 2 i-Size.
The Maxi Cosi ones take loads of different seats. Not only do you have the choice of Maxi Cosi's entire infant seat range (and I'd look at a much more up to date one than the Cabriofix, personally) you also have all the Cybex seats (not just the Z) and all seats by Nuna and Besafe.
Then also:
Recaro seats go onto the same adapters although their two which clip directly into the rotating toddler seat need interim adapters (I believe), which is a bit of a pain.
Joie seats all go onto those adapters as well apart from the Joie Juva, which is their cheapest/budget model anyway.
Axkid make an infant seat which goes onto maxi cosi adapters, using an interim adapter.
Cosatto have one seat which goes on - the Dock.
Avionaut seats go on too, they don't trade directly in the UK but they do have one seat they sell here, rebranded by Venicci - Venicci Ultralite. That's actually my favourite infant seat of all time and it does take a toddler seat on the base too :)
Thoughts on the three you've mentioned:
First of all I wouldn't worry too much about the age ranges, as these are only ever an estimate and different companies estimate differently. What you want to look at is the physical size of the seat/s and the weight and/or height limits. Most people tend to want to move on from the baby seat at about 9-12 months anyway, because these seats are very small, very reclined, low down in the car and babies of this age tend to want to sit upright and look around more. The carry feature also becomes less versatile, because most people find larger babies (9-10kg+) too heavy to carry in a car seat, so you don't tend to use it as much. Personally I think it's still useful for the sole reason of carrying a sleeping baby out of the car! But a lot of people switch early for this reason, so looking at the max limits of them might be misleading.
But anyway - the Uppababy Mesa has a height limit of 78cm which will almost certainly be reached before the weight limit. This will get an average sized baby to around 13-17 months, depending on when they have their growth spurts. If you have a baby who is very small for their age, they are likely to fit it until they are about 16-21 months. If you have a baby who is big for their age you would get until roughly 9-12 months.
Cabriofix doesn't have a height limit, but is very roomy - an average baby will get to 18-22 months in it easily. Small babies sometimes fit over 2 years old. Big babies still get a nice 12-15 months.
Cybex Cloud Z is another i-size one but with a higher height limit (87cm), and it's physically a similar size to the Cabriofix, IIRC, possibly even a bit bigger. Bear in mind that this is an extremely heavy seat, so might become unwieldy much sooner than the others, although the swivel base would help with this - just leave the seat in the car. Babies are likely to outgrow this seat physically before they reach either the height or weight limit, which makes it hard to estimate, but I'd say the Cabriofix ones are similar, possibly even a little longer.
Would not worry overly about the ability to put a second seat onto the same base. It sort of sounds like a good system until you get there. In reality what you've done is essentially decided which toddler seat you're going to buy already, which is very nice for the company (they get your £££) but not necessarily the best thing for you. Unless you want to research toddler seats now and you're convinced of which one you want later, I wouldn't worry about this. Another thing to think about is whether you think you're likely to have a second child, because obviously you can't use the one base for both toddler and baby seat at the same time. If you do decide to go for a modular system, it's well worth checking how both the toddler AND baby seat fit onto the base in your car. Fit in your car is important. It's a pain if you buy a seat and only later find out that the recline is so steep your child can't sleep in it comfortably or you only have 2cm of room between the passenger's legs and the dashboard.