Thanks for all your replies.
I phoned the AA line in the UK. They can issue me with an open ticket but if I give them any dates at the time of booking they'll be printed on the ticket. Once I pay for my ticket and it arrives (they said 1-2 weeks), can I then start reserving dates, or do I have to wait until I set off - really not sure about this part. I think I've read that you can only change dates once you've begun travelling - does the same apply to making reservations?
I'd ideally like to see the whole terms and conditions for a OW ticket. I've searched on the internet but no luck - is it freely available?
Thanks again.