Cookie,
I'm confused (for 2 reasons...)!
Firstly your quote of £86 return each. This doesn't make sense, that price doesn't tally with any of the tickets I can see. In fact the Saver Return from Hinckley to St Pancras via Leicester is only £42.20 so it doesn't even add up to £86 for 2! The Open Return is over £100 but you don't need that because you can travel on trains where the Saver is valid. The Saver would be useful because as long as you avoid the peak hour you can buy it on the day, don't need to reserve and if the return Eurostar is late you don't have to worry about missing the booked train and losing the ticket because it's valid for any off peak train for a month (and not even Eurostar have come in 4 weeks late!!). The Saver Return from Nuneaton to Euston is £32 but you would have the cost of getting to or parking at Nuneaton which woud soak up most of the saving and you'd have to walk to/from St Pancras, so it's probably not worth it.
Second confusion is where the advance tickets have got to. In fact they are still
not out for your dates, latest I can find are 4 weeks before you (August 4-7) on which dates you can get Hinckley - St Pancras Advance Singles
from £14. So that could be as low as £28 return (that's £14.20 cheaper each than Savers). If you want to try for these I'd keep trying over the next few weeks and see what pops up,
but these come with fixed reservations and if you miss the return train they might say you can't use the return ticket.
try it yourself at
http://www.thetrainline.com and just put from Hinckley to London (don't worry about which London or where via) and request a return journey - if 2 singles are cheaper it will tell you.
Incidentally, although it's no use to you now if you've already got Eurostar booked, Eurostar will sell you a through ticket from Leicester to Paris for £80.50
return in small numbers on certain off peak services. And coincedentally, I'll be passing through the Tunnel on the 10:00 to Brussels that day!!