Thanks for your post, Horseshoes.
I've got two suggestions for reducing your UK rail costs:
1. As Jno points out, you can often get (relatively) cheap 'Advance' rail tickets. On my sample date of 15 June, you can get from Bangor to London Euston, on a direct service, for £89.00 (leaving at 0618, arriving 0941). You'd then just need to use a contactless debit card to pay for the one-stop journey, on the Northern Line, to St Pancras International. (Alternatively you could take a taxi or, if you're fit, walk it. Euston is only just down the road from St Pancras). Search here for cheap fares:
http://www.nationalrail.co.uk/
2. (Quite possibly better). The Eurostar rail site allows you to book through fares from principal UK rail stations to major stations in France. (i.e. you can book from Bangor to Poitiers and then just pay for the bus for the last bit). On my sample date you can leave Bangor at 0618 and arrive in Poitiers at 1740 (combining the information I've provided across two posts here) for a total of £127.90
http://www.eurostar.com/uk-en
You can still book the last section (by bus), on my SNCF link above, for a tenner. So that gets you from Bangor to Parthenay for a total of £137.90 (plus your London Underground & Paris Metro travel).
Remember that you need to book early to get the cheapest fares though!