If you have a service due and they stock the oil you need, you can get 50% off from Tesco.
https://www.tesco.com/groceries/en-GB/search?query=castrol&icid=tescohp_sws-1_castrol
https://www.tesco.com/groceries/en-GB/search?query=castrol&icid=tescohp_sws-1_castrol