Equally, with one sleeper a night or, at best, two day trains to Barcelona, there is limited flexibility on arrival / departure times by train.
And fares won't be cheap. Being fortunate enough to get free rail travel, including Europe, I only have to pay reservation fees on the TGV and the staff rate on Eurostar. And yet it is STILL cheaper by plane to Barcelona, and 6 hours quicker, with many more departure time options.
My two cents is that Eurostar will aim for destinations with substanital business flows within the 5 hour zone where the train can compete with air on journey time+hassle factor and get away with charging a premium. So Lyon, Geneva, Bordeaux (when the LGV SEA opens), Amsterdam and possibly Koln / Dusseldorf / Frankfurt. There might be the occasional special to the Med, but they would be one offs. (is there a plural of one off?)
However it all depends on the UK Border Agency being happy with arrangements, as there is a rumour that the DB plans for Koln have been delayed (again) for this reason.
I think they've been delayed due to delays in the stock arriving. I think Eurostar may have also mentioned Cologne at one point too. We'll see - it'll certainly be very exciting.
Eurostar already run to Avignon, which is basically the Med. It'd be prudent to start building this up to increase awareness - either through frequency (it's a summer only, once weekly return I believe) or by gradually extending it. It's a railhead but I imagine Aix and Marseille would also be popular. Perhaps Aix as one-way/set-down only so no need for passport stuff, initially.
And a Barcelona service would definitely stop at Perpignan and some other popular locations like Nimes, Montpellier and Narbonne, which would have shorter journey times than Barcelona.