Complex sentence: It was then abolished by the Finance Act 1919 after several years of steady petrol price rises and replaced by vehicle taxation, and the tax disc based on horsepower, after which the cost of petrol was about 4s (£as of) per UK gallon.
Split into simpler sentences: It was then abolished by the Finance Act 1919 after several years of steady petrol price rises and replaced by vehicle taxation, and the tax disc based on horsepower. Immediately after the tax was removed petrol was about 4s (£as of) per UK gallon.