You are in for a woderful adventur and the connections are easy, when you know your schedule you can book up to three months in advance and on occasion some great discounts pop up.

ONE WAY -
CARDIFF CENTRAL to WATERLOO London - 2:40 minutes fare super saver 40 GPB

http://ojp.nationalrail.co.uk

ONE WAY
WATERLOO London to Lorient France 5:30 minutes cost 160 GPB
http://www.eurostar.com

(I would "chunnel" ANYDAY rather than fight the airport mess.)

Sincerly,
Gargoyle