Most garages will save a couple of quid anywhere they can, not all, but most that I've dealt with. One problem is they often don't really understand like for like, Carlube 5w-40 is a pretty basic 5w-40, Castrol Edge is one of the better ones, just below the top end ester based oils, but all they see is synthetic 5w-40. Plugs come in all different qualities too, just have a look on Ebay, the proper NGK/Denso/Bosch ones are about 5 times more than the Chinese rip offs. Yes, a bit of it is paying for the name, but part of it is also paying for the metal (iridium/platinum/copper etc) used in the plug. All they have to do is use a thin layer of iridium and they can then sell it as an iridium plug. The decent iridium plugs will use a good thick layer of iridium so the plugs last a lot longer, spark better etc.
Ignore the name on oils where they call it a Diesel oil, they are fine to use in petrol engines too, they just put Diesel on the label to give customers with diesel cars confidence that the oil is suitable for diesel engines.