Whenever you buy ANY new item from a retail outlet, you are covered by the following acts...
Sale of Goods Act 1979.
Supply of Goods and Services Act 1982.
Sale and Supply of Goods Act 1994.
The Sale and Supply of Goods to Consumers Regulations 2002.
The above does NOT cover items purchased over the phone, by mail or over the internet.
Basically, you have the following rights :
If the goods do not 'conform to contract', (IE, they must be as described, fit for the purpose and of satisfactory quality) then you have the retailer by the nads.
Much of the law is subjective, but can be used in your favour, if you can apply it correctly.
It is ALWAYS the Seller NOT the Manufacturer, who is responsible to correct any faults.
There is NO set time in law, whereby you are entitled to an automatic full refund. The law just says, within a reasonable time. But if the item shows signs of being faulty within a short period after purchase (usually 14 days, but almost never after 28 days) you are entitled to a FULL refund, as there is a supposition that the item had a fault from day one or manufacturing and is therefore not fit for the purpose.
But as this is a very time limiting period in law, you need to act swiftly to invoke your legal rights, should this occur. Also, if you have had some benefit from the product, that could be used to reduce any refund offered by the seller.
In English law, there is a six year window for 'damages' and in Scottish law it is five years from discovery. This means, that if the item proves to be faulty within the first six years, you are entitled to have the item repaired, by the seller at their expense. This is a maximum time limit not a minimum and will vary depending on the item and what would be deemed as reasonable.
For instance, it wouldn't be reasonable to expect a watch battery to last 6 years. But totally reasonable for a TV or other similar home appliance.
If the seller refuses to repair the goods, then you will need to bring a case within the Small Claims Court, against the seller.
So, why buy an extended warranty, unless they offer something like in-home repair or a new unit in case of a fault. But if it is just to cover repairs, there is no need, you are covered in LAW.
TIP:
When you purchase any item, always try to purchase the item using a Credit Card, as you are given extra protection under the Consumer Credit Act. Never use a Store Card, Debit Card or Charge Card, most of these offer no extra protection under the Act.