If you are thinking of travelling to UK check out this crowd:
I have to point out I have never dealt with them personally but one thing is for sure you are definitely their target market. They have a good selection of M3s all in one place.
I assume the OP understands the lack of a roof can compromise the handling of the car as he drives a Z4. What I would question is would the compromise be really that noticeable in real world driving?
Are comments relating to the poor driving experience really only down to too many people
I know that there are some around here that think convertibles/cabrios/soft tops are for girls but to be honest you'll normally find that males drive the majority of convertibles on our roads. Personaly speaking I can think of no better cure for a hang over than driving with the top down!!