Sadly, you underestimate the time it will take to get from Bournemouth to Cornwall and back by road. There are no day trips by coach and, while it may theoretically be possible to get to Cornwall and back in a day by train (I haven't checked), there's no direct rail line to the west and you'll spend all the day travelling. As an idea of how far you can travel from Bournemouth in a day on a coach excursion and have some time to spend at your destination to enjoy it, try this link:
Thank you for the information.....we may have to arrive earlier or extend our time in London, and try to find a short 4-5 day coach tour of Cornwall/Devon from there.........the day trips from Bournemouth look interesting, so we may be able to book a couple of those while we're in Bournemouth. Best regards, Maureen.
In response to comment #1: Bournemouth to Penzance is more like 5 - 6 hours, so really not a doable day trip.
To put it into a Canadian perspective: just imagine yourself in Vancouver. Would you contemplate a day trip to the Icefields Parkway?
Thank you for your reply, and the great info.........they all sound like lovely places to visit....we will have to rethink our schedule...best regards, Maureen.