Upvote:4
If your question is about which visa to apply for first, it makes no difference, however I would suggest you apply for the UK visa first as it seems to take less time (3 days) on average to process than the Schengen visa which requires you to go submit biometric data and can take up to 15 days.
As for the travel reservations, it's enough to show them proof of how you're getting there, in your case from the UK a train or flight reservation is enough. They also want to make sure you're leaving the Schengen area, so again a flight out of France should be sufficient.
If you're going back to the UK after Schengen, just make sure your UK visa covers that and you have another flight out of the UK. This is really all they want to know, that you're not planning to stay.