Consider the new high price of petrol in the UK?
Consider the new high exchange with the dollar?
Consider the traditionally lower price of petrol in the US?
What relative price/exchange rate and volume would we have to see for it to become cheaper to buy petrol in the US and ship it over to the UK, than buy it on a Shell forecourt?*
* Yes, I know we wouldn't be ABLE to do it - its hypothetical!