Hi, best place to buy oil is from a reputable sellers like opie oils, Eurocarparts, but you can buy through eBay and amazon, you will get best prices there. www.carparts4less.co.uk also have good prices, it’s basically same as eurocarparts, Shell helix ultra 0w30 or 5w30 but make sure is ACEA C2 or C3, as already noted by other members. This is important for the life of your DPF if fitted one and the catalytic converter. If you use C2 you may get better fuel efficiency but may get higher oil consumption where if you use C3 spec you may have slightly lower fuel efficiency but also lower oil consumption. The difference in fuel consumption it’s absolutely minimal. It’s up to you which one you want to try but both are suitable for your car. You can get 2x 5l and have some extra in case you need to top up. 5w30 C3 is cheaper option too.
More for the oil types here:
https://www.opieoils.co.uk/t-acea-car-engine-oils-specification-explained.aspx
Good luck