For the Casse, disband all of your units but one swordsmen, then just build up for a while until you can afford a few more troops, then start conquering Britain. Make sure you don't build roads until you're ready to defend yourself though. One of the rebel stacks around there always seems to attack as soon as you have roads built. When it comes time to invade Europe, I find the northwestern coast of Gaul to be a great invasion site. If you go too far east, you'll get sandwiched between the Aedui and the Sweboz.

For the Aedui, just blitz the Arverni and go from there. The AI, I've found, doesn't really know how to use Celtic troops very well, for some reason, so none of those big battles at the start of a Gaul game are very hard. Just line your troops up and flank, and the other army will basically rout in a minute or two. After you defeat the Arverni, you could take the western part of Gaul, then move on to southern Gaul and eventually Rome. There's no need to migrate to Britain in an Aedui game unless that's really what you want to do.