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If it is only a couple of litres, I would suggest buying a 5L of fresh oil from a supermarket.
£4 for 5 litres:
https://groceries.asda.com/product/oil/ktc-extended-life-vegetable-oil/910000322461You can still get some from takeaways, you just need to see what their weekly oil change day is - the grubby ones (chicken wing types, just not Morleys), will leave them at the back for the "oil recycling" company to pick up. You can pick up a 25 litre barrel of used oil.
Are you going to use to my your own biodiesel? You can run a non-HDI engine on 50% with no issues in the summer. Remember to filter it it well.
Where can I get a couple of litres of used cooking oil? Are takeaways allowed to give it away?