Don't worry, Ariadne, it can't be easy trying to co-ordinate a meet up over such a wide area!
I'm obviously going to say Plymouth is easiest for me because I live there! But, seriously Exeter would be fine. Okehampton might be an idea, maybe more central for a lot of people?
And anyway, I will say here that my daughter will be having her first baby in four weeks' time and as she'll be a single mum, she's coming to stay with us for a while before going it alone. So it could well be that a) I'll be too busy, b) I'll be too tired, or c) both!!!