Britons are the only barbarian faction who can build temple of trade (Brigantia IIRC)

However, to trade, you'll need trading partners, in the case of Britons, that means getting trade rights with either Gaul or Germany.

In a campaign way back (I was still playing 1.0 at the time), I sold Belgica to Gaul. and got trade right with both neighbours.

Without any continental provinces, I was pretty sure they wouldn't wage war on me soon and that they'll stay my trading partners as long as they kept fighting.

With the money, I developped the island and sent an army to conquer Ireland.
Then, I turtled a bit, built a fleet and a decent army and sent it to take over the spanish peninsula, beginning with Asturica.

Once Spain is yours, you'll have mines, friendly barbarian settlements which are easy to defend thanks to the mountains and... access to the Mediterranean.