I have used the Britax Freeway, £80 from Toys * **, this fits fine, I have had it in the 200SE and now in Sportcross, the secret to a tight fit is to pull the seat belt out all the way until the ratchet engages, then put downward pressure on the seat with your knee (not whilst child is in it), pull the strap as tight as possible and engage the strap lock on the seat, I can ensure you if you do this the seat is secure and is going know where.
With regards to ISO Fix, I spoke to Britax and the latest ISO fix car seat is compatible and approved but for £250 quid seems an awful lot when there are others that are just as secure.
Britax web site has all the contact details and they are really helpful.