Three points:
1 if you buy goods that are sold at reduced or sale prices, it may well not be possible to return them, so if you buy someone a present at a reduced price, you need to either be very certain that they will want the gift or to check in the shop that they will accept it back.
2 the online stores, whether run by businesses that do have an ordinary shop or not, have far lower overheads usually than a store in town. They may well be operating out of a warehouse that doesn't have to look smart, so it makes sense that they call sell wares cheaper than a shop that customers go into.
If they haven't made it clear that an item is cheaper online than in their stores, complain. I believe this is a contravention of the goods' description act
3 It should be possible to notify firms that you no longer want to receive e-mails with offers from them. If their homepage doesn't have a facility for doing so, simply block their e-mails so they are sent directly to your junk (spam) folder.