I have been through this problem and the best way round it is to order the lot together. I have the Britax Zeta, which is compatible with the Isofix base. I mistakenly bought a Britax cosy tot car seat and isofix base and then tried to buy the pushchair frame, it is not adapted, though Britax can do it for a small fee.
Britax were crap at giving the correct info and i told them so, and as they advised me which base to buy, I was furious as it was the wrong one and they did nothing to help put it right. I told them I would never reccommend them again.
Tip: there is a sticker on the base and seat, a 'T' or an'S' and both must match for the carseat to fit the frame.
If you are interested, I could sell you a barely used ( probably half a dozen times) complete Britax Zeta system, in Cinnamon, very nice for girl or boy, car seat, isofix base and pushchair, all compatible! I used it for a 3 week old bay, who's mum had PND and I needed to pick the baby up and get out and about, but my own pushchair was from 3 months upwards.
If you are interested, I coculd send it, email me,
[email protected]