£200 for each seat correct? The Joie 360 is £200 by itself :)
The Stages is a nice seat but if your baby is big for his age, the harness won't last him - it's only up to 18kg which is age four average - for bigger children this is more likely to be age 3. Then it's only with the adult seatbelt beyond that which most people feel 3 is a bit young for.
If it is £200 for both seats then I'd get 2x Stages or 2x Joie Steadi and plan to save up for a larger seat when the oldest outgrows the harness.
Cars past about 2006 or something all have isofix so that will be fine if you are getting a fairly new one. Do check fitting lists for seats before you buy/before you settle on a specific car. Occasionally you can get into issues.
Assuming you mean £200 per seat:
For the new baby I think the Joie 360 is an excellent choice. You could also look at Britax Dualfix 2R. Both seats are suitable from newborn and can be used rear facing right up to 18kg. The baby can then move into the older child's seat as he will likely have moved into a booster seat by then.
For the toddler I would look at Diono Radian 5 and Britax Two Way Elite. Both of these seats can be used rear facing up to 25kg, which is about 5-6 years for a bigger child. They fit using seatbelt, but for extra stability they lean on the seat in front (so pick a seat you're less likely to move back and forth) and use lower tethers to secure which wrap around the seat in front. There are good fitting videos online or some online retailers will give video support.
I wouldn't recommend using the Two Way Elite forward facing, but the Diono can also be forward facing if you prefer later on, also harnessed to 25kg, but it needs top tether to fit in this way. Top tethers are standard in cars newer than 2013 but any older and you would have to check manually for it. The Diono is also flight approved which may be handy if you ever want to take a car seat for a toddler on a plane. The downside is that it is a little more complicated to install and use than the Two Way Elite. You will probably want the infant insert for the TWE.
Two more seats which are slightly over £200 but sometimes come down in sales:
Axkid Move
Britax Multi-Tech 3
These both have a bit more side impact protection and don't need to lean on the front seat so you can get a bit more flexibility there. The Axkid can't be used forward facing and the Multi-Tech is only forward facing with harness until 18kg, but the safety and comfort in them is very good.