We are moving to Italy and will need three car seats. The Jetta is very popular over there. We did some shopping when we were over there last and found that Europe has LOTS of more economical car seats. Most were by the same makers as we have here. Some we European makers, but that sell here as well. You might get on some websites and see if you can special order. The prices in the store seemed comparable to what you would pay here. The British websites would probably be the best (usually end in .uk instead of .com). Sometimes you can effectively translate a website, but our experience is that the website doesn't work right and you loose links. I'm sorry, but I don't remember the names of the companies we looked at. My husband will be there before the kids and I arrive, so we are just going to buy them there.
One feature that many of the seats there had were the sides adjusted, so while you have two kids, they could have more room in their seat and then you can make it more narrow when you need. You might keep in mind, too, that your son will be going into a booster soon, and they are more narrow, even with the back if you get one like the Graco that the back comes off for when they get older. I hope you find the right seat!