You can book each section of your journey on the Eurostar website. Typical fares:
London to Poitiers = �178.50
Poitiers to Angouleme = �13.50
Angouleme to London = �224.50
However Eurostar offers far cheaper fares, for London to Paris (and vice versa), when you book a return journey (rather than booking separate single journeys). By booking the international journeys with Eurostar (i.e. as a return journey to and from Paris) and then booking the continental journeys with SNCF's UK website (which is actually a portal for Rail Europe) these fares are typical:
London to Paris = �29.50
Paris to Poitiers = �22.50
Poitiers to Angouleme = �13.50
Angouleme to Paris = �20.50
Paris to London = �29.50
http://www.eurostar.com
http://www.sncf.co.uk/
If you decide that you'd rather speak to someone to help plan your journey, contact Rail Europe:
http://www.sncf.co.uk/aboutus/contact_Us.asp
Chris