Strategically speaking, try to hit the Britons early on before they can field any of those chariot archers. In my current Julii campaign on H/M I've found the Brits a bit of a pushover. They hadn't expanded beyond Samarobriva (the Germans seem to have fought a successful defensive campaign), and once I'd kicked them off the mainland I just blockaded Londinium. They dont have any other ports, and with no trade income I can just sit back and watch them starve.

Guess where my Senate missions have me involved now, though? Pontus! Oh joy, more chariots...