As Britain you should be able to handle the Gauls and the Germans but defeating Rome is a challenge. As stated above, ally with Germania and sell trade rights and map info to Gaul before you attack them in Brittany. You'll fight a lot of battles around Aleisa but you should be able to defeat them in detail. Keep your diplomats moving out to sell Alliance/traderights/map info. Aim high. I sold an ATM package to Carthage for 9600 dinar (hard/hard) and it was my first offer! Use this cash to bribe Gauls or rebels and life will be easier.

After the initial Gaulish attack I went back an took Tara. I concentrated on trade there as well as the north of England and my unit production in London and the Continent. Make sure to build the right temple. One increases trade, one produces Woad Warriors then Head Hurlers, and the third Druids.
Trade for the home front/coastal areas and a mix of the other two on the continent. Keep your fleets on "skirmish mode" the gauls have too many to start with and so it's more efficient to ferry troops and family members across the channel and run up north to hide.

Tactics early on are rather basic. Use light chariots to disrupt their formations, they will chase you, and then attack with war bands center and Woads and Swords on the wings. Use the war cry and overwhelm them paying particular attention to killing their leader. War hounds are good to chase routers/disrupt missile units as are barbarian cv.

After you have taken greater France you'll have to make a strategic decision. Where to? In my game I had three options. The Julii were fairly weak and had taken only Segesta and the northern Spanish province. So I could take them on, or continue after the Gauls in the Po valley and Spain, or attack the Germans before they hit me. I was tired of the Gaulish fleets harassing me and figured it would be good to take Milan and venice before the Romans did.

WRONG. As I went for Milan the Julii came north and the Germans got in on the game. With 20/20 hindsight I should have really knocked the Germans for a loop and let the Julii take the Po valley. I used a lot of cash bribing Roman armies but my dinar would have gone farther in Germany. Take out the German capital but anything east of that loot and leave it to rebel, you can always get it later.

After Germany its on to Rome. A whole other story.