Markup on electronic parts from the more sucessful online places will be around about 10% (or was when I was working in that area) more on some stuff less on other stuff. Places with a high street presence will often still be 40 odd percent markup but if thats a large part of their income they will struggle to be competitive (or even survive).
So, potentitally, if the failure rate on a fairly expensive (say £400) item is 5% in the first 4 years, and the manufacturer only offers a 1 year warranty, it does put the retailer in quite a difficult situation if customers decide that they don't want to pay for the repair, or replace their £400 item just yet. A portion of the profit they made on those items may well have to be spent on repairs or compensation if customers invoke the SOGA.
I think that the consumer electronics retail industry relies on customer's lack of awareness or willingness to use the SOGA. In the years that I was more closely involved (mainly repairing items, but also selling in small quantities) in consumer electronics, I witnessed many changes. As complexity of products rose, so did repair costs. At the same time, prices seemed to continually fall, I assume both at the factory gate, and in the shops. This was fairly good news for cunsumers, but it wasn't all good news. As the market was so competitive, the quality of many items fell to a level where failure rates were fairly high. I used to get no end of TVs, Satellite recievers, VCR etc in for repair, simply because the manufacturer had cut too many corners. Usually the cost of repair for the consumer wasn't too bad, so going down the legal route was not really worthwhile. I used to feel sorry for customers who had bought fairly expensive items, and the cost of repair was beyond economical. That's where the "sheds" (Comet, Dixons, Currys etc) would have said "should have took out an extended warranty. And I think that's where the idea of the SOGA comes in. It's to help protect consumers from having to pay to protect themselves from further expense if the item they pay a lot of money for fails.
I recently assisted a customer in the purchase of a TV (for around £600). I went into the store with them to help them choose. Once decided, they purchased the TV, and then came the inevitable offer... "I have to tell you that if this TV goes wrong, it could cost a lot of money to repair. For just £199, you are covered for an extra 4 years." I said "so are you saying that these TVs aren't reliable ? Anyway, the SOGA should help if the TV has any serious problem withing the first few years". Then came some explanation that the SOGA doesn't protect against much, and that the TVs are relaible. I advised my customer that if the TVs are relaible, then the extra £199 is a bit of a waste of money. In the event of a fault, we'll find out if the SOGA is really pointless.