We must try and be realistic. Only in July the price rocketed to $147 per barrel. Very recently it has plummeted to around $50. Incredible change in very short period indeed. Unprecedented.
First remember that many will buy their oil weeks or months in advance to ensure supplies etc. It will be that sort of time before their costs reflect the drop in world prices.
Even for those who buy oil at spot price (today's price) remember there is a 6 week lag from world market, till the oil goes to refiners, through the refining process, and gets delivered to the pumps. So a bit of patience and you do see the drop.
Allow for the change in the exchange rate of the £ against the dollar.
Allow for fact that most of the price is tax. So the world price only affects part of the price.