I got my P38 to accommodate the expanding family.
Enough room for bikes, prams, camping holidays, and all the other crap. I'd keep the P38 if I were you. Best family car I've ever had. It's the only car we've got, and the only one I can ever see me needing.
The leather seats are wipe clean for when the little darlings pour water, puke and crisps everywhere, and as all P38's are a bit old now, you won't get so upset as you scratch it lugging heavy stuff in and out of the boot.
ISOfix has never been an issue, just use the seat belts to secure your baby seat in the traditional fashion. I always thought my seats were just as solid using 3 point belts. Not tested this in a full speed collision yet though.
Also go and buy a Maclaren folding buggy to "keep in the boot". That way you'll be ready when the silver cross pram get put in the garage "just to get it out of the hallway" and never used again. I've got one in my mums garage (still) if you want another?