There is no need to retrofit ISOFIX for a car seat at all. I have a 14 month old son and his car seat fit's perfectly into the S40 rear seat and lock's in tightly with no movement at all using just the seat belt.
I bought this from Halfords
http://www.halfords.com/webapp/wcs/s...egoryId_165763
This will fit my son up to around 4 years old depending obviously on weight and size, generically fitting 'up to 4 years' is only a guideline remember.
I dont beleive this is a rear facing car seat that will go up to 4 years old, however that seat above WILL fit rear facing, my son's already been that way, it's a lever underneath that you can change to be between forward facing and rear, the Halfords lot do tend to be quite clued up on this which is remarkable considering other thigns thay havent a clue about.
Your best bet in my opinion is the above form of seat, not cheap but will last. It doesnt nessasarily need to be Britax but they are one of the most awarded makes of car seat. Also for a 20month old please do not take offence but I wouldnt have thought it wouldnt be suitable beign rear facing now, I think it's around 9KG that you are legally allowed to have a child facing forwards so your child must be over that now if my son is...though he is a big lad for his age.
I can take picture's of the seat fitted if you want any further proof but I highly recomend this seat, my son sit's comfortably, the 5 point harness will secure him enough but allows him to kick his feet, move his arms etc like holding a drinks container with ease.