Just been into my local tesco express store who have the following mobil engine oils reduced with a yellow price cut shelf label.
Mobil Super S 10w40 Semi Synthetic 5Ltr - £11
Mobil Super S 10w40 Semi Synthetic Diesel 5Ltr - £14
Mobil Super M 15w40 Multigrade 5Ltr - £11
Mobil Super 1 Performance 0w40 Fully Syntheitic 5Ltr - £16
Apologies if not nationwide. Not sure if available online.