Rumours are around that there could be a new direct rail link between London St Pancras and Bordeaux avoiding the need to change in Paris – at some point in the near future. According to a a report in the Independent (4 May 2017) the way has been cleared for the project to be realised, subject to some technical issues between the French and the British authorities and the railway companies involved.
This could be a major improvement in time and costs for anyone wanting to visit Bordeaux from the UK- and certainly make the option of flying less attractive – consider the time, hassle, cost and environment factors involved,. Meanwhile you can fly or train go by with a change of stations in Paris.
As of 2018 the Train and Eurostar takes about 5½ hours allowing an hour to transfer from Gare du Nord to Gare Montparnasse in Paris. It usually takes only about 30 minutes by public transport. depending on time of day.