As the Brits, you are not badly off. Conquer fast and furious and you'll be fine.

Do not go after rebel territories but instead after the Gauls. Also build mines for steady income. Londonium and Sambovica is also a nice port pair that generates lots of cash.

Brits have it easy compared to Spain or Dacia financially.

Don't forget that extermination is an option to take you out of the hole quickly. But population maybe more valuable.