In my opinion, a good place to get name-brand components is from a distributor like Parts in Motion. Filters supplied by BMW original equipment manufacturers (OEMs) Mahle, Mann, and Hengst. Once again, it's a popular brand of oil, and many of them are BMW LL 04 spec. Oils by Opie are reasonably priced.
Does Halfords offer very low prices? Maybe I'm being too pessimistic, but I seriously doubt it.