Halfords have 4 litres of their own brand oil for £10 at the moment. Same price for part or fully synthetic 5w-30 and 10w-40 part synthetic. Don't know if it's available in store or just online reserve and collect.

The website quotes "our lowest price ever". Halfords have been around for 50 plus years, so I'll take that with a pinch of salt. Still a good price though.