I have just spoken to Britax, who manufacture an IsoFix baby seat, to check their use of the Duo IsoFix seat in the Mini.
They said that since it had not been approved for use in the Mini (BMW must crash test the seat in the Mini), it would actually
NOT BE LEGAL to use their seat in the Mini. They said people would have to badger BMW to get them to approve it.
The only one available, then, is their own baby seat. At £275+VAT, that is very expensive.
Tim (& MeloYelo)