You can book through tickets to Poitiers on the Eurostar website:
http://www.eurostar.com
Booking isn't yet available for next summer but looking at the prices for dates closer to hand shows that just £43 each way (ex St Pancras) is the standard fare available for early booking. (If you're travelling from somewhere other than London, you can get a through fare on the Eurostar website from all major British stations - it usually works out far cheaper than booking the UK component of your travel separately).
The Eurostar website shows times faster than those indicated by Mushroom25, with outbound journeys taking about 5½ hours and return journeys of around 6 hours. However their recommended route requires you to transfer between Gare du Nord and Gard Montparnasse in Paris. (Take Metro Line 4 to Montparnasse–Bienvenüe and then transfer to Montparnasse proper by using the moving walkway).
There are 132 campsites in Poitou-Charentes listed here, with reviews and links to their websites:
http://www.ukcampsite.co.uk/sites/results.asp?p=1&county=Poitou-Charentes
(Note that my link goes to the first of 5 pages. Most of the campsites in and around Poitiers seem to be on page 2. You can zoom and drag the map and then hover your mouse over a map pin to see which site it marks).