It isn't the size of the seat that matters (the UK tends to group seats for different ages), it is that your child is in a seat that is suitable for his weight, And that he isn't too tall for it (unless your seat is Scandinavian, a rear-facing seat is outgrown when there is only 2 inches left from the top of the child's head to the top of the shell, or when the child's shoulders are 1 inch above the top slot for the shoulder straps, and I think a forward-facing seat is outgrown when shoulders are above the top slot for the shoulder strap or the ears are at the top of the shell). So what you need to do is to find some car seats that go up to your kid's current weights (and preferably slightly more), it would really be best for your youngest to be rear-facing, and shouldn't be thet difficult to find. Depending on the size of your older child, he would be better off in a rear-facing seat, but if he is a bigger child, it may be more difficult to find. You want a convertible seat, and and you may need to look at seat imported from the US or Scandinavia, as even your youngest may weigh over 13 kg (the limit on most convertible seats for rf in the UK, as they like to turn kids ff at 9 kg there).
Britax has some great seats, but they're Scandinavian, all the UK Britax seats only rf to 13 kg. Here's a list of places that sell seats that rf past 13 kg, and some info on erf in the UK, think:
http://www.rearfacing.co.uk/buyersguide.php
The great Scandinavian seats, by the way, are:
Britax Hiway, Multitech and Twoway
Acta Graco Duologic and Belogic
Brio Zento
BeSafe iZi Kid
MaxiCosi Mobi
(although not sure how long-lasting the Besafe and the MaxiCosi are, in regards to height and weight. Britax Twoway and Multitech, Acta Graco Duologic and Brio Zento are known as the ones most likely to fit older kids, 2,5-5)
