Oil or petrol in the forecourt has not fallen for several reasons
1) The two transactions are involved with oil production, foreign exchange Dollars Vs Sterling and the cost of a barrel of oil
2) The Oil companies hedge their production by doing a financial trade called an EFP The know what they will produce in 12 month time so the sell it forward on the Futures market and then do the same with the Foreign exchange so that they can budget their costs for the future. In twelve months time they buy the future contact back and having drilled the oil for delivery and the forward Foreign Exchange has been covered at the time with the Future contract.
3) The production refinery costs tend to be in sterling e.g. North Sea oil to the Shetlands, workers and costs are sterling based.
4) In theory then the price of oil should fall in twelve months time, though it probably won't.
5) The Government tax it at such a high level, that the tax is the major component in the cost of a gallon of petrol.
6) Opportunity the oil companies can see a wonderful chance to make bumper profits in 12 months time it is unlikely that the price will drop