This could work. You could also check out the EasyJet option and fly from London to Paris as this could work out cheaper, although I am guessing that the timing would be the same, ie, by the time you have checked in for your flight, you could be half way there with the chunnel.
The Eurostar is great, but it can be expensive, so book ahead of time.
www.seat61.com is a great way place to look at for trains.
Also consider a coach - you could do this journey all by coach but the time will be huge compared to flying/rail. about 8 years ago
There is a train from Paris to Barcelona, but check budget options as well as they could be significantly cheaper. keep in mind that budget airlines (RyanAir, EasyJet) operate outside of Paris and Barcelona (in Girona near Barcelona). You cold also rent a car and drive across the very beautiful Basque region and through Androra - but of course that would take more time! about 8 years ago
The train from Toulouse to Barcelona passes over the Pyrenees, it is a very scenic route. Of course you'll get to see alot more travelling by train than being up in the air the whole time, i've never done it but I don't see anything wrong with your plan. about 8 years ago