Who would want to do that?
Not only that, the Marseille train leaves London at 07:19! Great for people in London, not so great if you live outside. Firstly, it might not be possible to get in that early, and secondly, mid-week, peak fares in to London are extortionate.
I checked a fare in October (Fri-Fri, before half term): £178. Sat-Sat in October: £129.
Flying: £110 return with Easyjet, including baggage. Or from Heathrow with BA: £114. At more reasonable times of day. It's a 2 hour flight.