Edit: on closer inspection (thanks Krist van Besien for the seat61 link) it seems that sadly, the only practical options really are coach with ID or Eurolines (cheap, £20-£50ish depending on advance booking, but 5hr+), or Eurostar.
Train > Ferry > Train
Many possible routes, here's the fastest:
So that's 4 hours plus interchange time, costing around £85, Central London > Lille, prices seemingly not dependent on advance booking.
Edit: The interchange times are horrible: the ferry operators have some savage policies that really slow this down. 45 minutes for foot passengers to check-in (compared to just 30 minutes for vehicles), plus infrequent shuttle buses between ports and stations at both sides that don't seem to match up with either the train or ferry timetables. seat61 recommend allowing an hour at both Dover and Calais for UK to France train-ferry-train journeys, pushing the total to potentially 6 hours. Unless these policies change, sadly this option is more expensive, complicated and slow than a coach.
There might be combined "train and ferry" deals out there that lower the overall price and help you find fast connections, but I couldn't find any clear info. There are "rail and sail" tickets to Ireland and the Netherlands, but not to France. South Eastern Trains once offered a combined "day trip" ticket to Calais, but these seems to have been dropped - apparently for a while it was telephone-booking only, then even call centre staff stopped selling it.
Train > Coach > Train
Two fatal problems here:
- Looking at the (Eurolines) timetables it doesn't seem to be faster than ferry. They mostly take the coach by ferry anyway (total 2 hours ish), and in the rare cases when they don't (which don't stop anywhere in the UK except London except on the return journey), it takes about an hour and a half to get from Calais to Folkestone (near Dover), so is barely faster.
- Despite driving through Dover and Folkestone/Canterbury, neither Eurolines nor ID's online booking tools allow you to book from these towns. Seems they don't stop there. They want you to get a coach from Dover to London, then another coach back to Dover on to Calais...