Halfords 5w/40 is made by Comma (owned by Exxon/Mobil) and is Comma Syner-G 5w/40 fully synth and meets ACEA A3 B3 API SL CF MB 229.1, VW 502.00/505.00, BMW Longlife 98, Porsche industry standards just like the expensive stuff.
IMO, as long as the oil meets those standards and the requirements stated in the manual for your car, I see little, if any, benefit in spending money on the more expensive branded oils, especially if the oil is changed frequently or if you're only gonna keep the car a few years. Yes, there are oils that use the more expensive PAO/Ester base stocks, but are you really going to notice the difference? Has anyone experience engine failure because they used a cheaper brand of oil even though it met the cars requirement? Anyone noticed faster 0-60 times? There are no guarentees that the expensive oils will noticeably out perform the cheaper stuff.