So I found this website https://ukla-vls.org.uk/ of an organisation that tests oil independently and publishes the results. Specifically they deal with claims where the oil specs claimed by the manufacturer are not met.

Interestingly there have been a few cases of oil supplied by Westway not meeting the claimed specifications. Although there does not seem to be a case relating to 5W30 JLR spec oil it does make me wonder if I can trust Westway (and other small suppliers) more generally and should just stick with Castrol to be sure it is to spec.