The only one that can do exactly what you specify is Joie i-Level.
£175 for seat + base and the base takes the i-Quest toddler seat.
However I'd ask you to think ahead as to whether you plan to re-use the baby seat while you still have a toddler in the i-Quest. It's a nice tall seat, so will last an average sized child until around 4-5 years, if they are large for their age: until approximately 3.5 - 4 years old, if they are small for their age, possibly until 5-6 years old, depending on weight.
If you think that's likely, then you'll want to plan ahead to either purchase a larger toddler seat for the older child, possibly a 25kg one from roughly 1-7 years? Or a second base/set of bases for the new baby to use the seat on. Note the i-Level is unusual in that you can't fit it with a seatbelt for the times you're not using the base (most baby car seats you can do this).
It also means you're cutting some options for the second seat off, for example, you might later want to look at swivel seats, or a seat which reclines more, or a seat which gives you more room for the passenger, or one that gives more legroom for the toddler. I'd also generally advise that it's a good idea to have a fitting for the seat (even if you don't actually buy it yet) in your car to see what it's like to use, which is obviously not possible at the moment.