"How long before fuel rationing?"
I believe that I'm correct in saying that the last time there was fuel rationing in the UK was back in 1973/74. I remember that my dad had a special card to allow him to get petrol as he had a company car that he used for his work.
In October 1973, Arab oil producers placed an embargo on countries led by the USA over their support for Israel during the Yom Kippur war (this was a surprise attack by Egypt and Syria during the Jewish holy day of Yom Kippur). That policy came alongside a co-ordinated cut to oil production.
This meant that oil prices quadrupled very quickly indeed. Not just going up a bit, but think about petrol prices going up to £4 per litre nowadays.
Currently, the UK gets most of its fuel from from the USA followed by Norway. The routes from these countries to the UK are not affected by the current war, however the price has increased due to the global nature of oil pricing.
Just to add an additional point, oil prices fell back a bit after the end of the Yom Kippur war but they then further tripled again in 1979, after the Islamists gained control of Iran in the Iranian Revolution. This time there was no rationing in the UK but there were very long queues at petrol stations and some petrol stations did run out of petrol.