You have seven days from the date of delivery within which to examine the goods, and if necessary to notify the supplier that you wish to return them/it. If you opt to return you do not have to provide a reason.
Once the seven days has elapsed and you've tried out the item you may still return it, but only if it isn't 'as described', or if it's faulty. If a fault manifests itself within 6 months of the date of purchase the law assumes that the fault was present at the point of sale, and the supplier must replace or repair the item within a reasonable (but undefined in law) time.
Here in the UK we have the best consumer protection legislation in the world, and you need fear nothing that a supplier decides to include in pre-sales terms and conditions.
Finally, you're protected by the Sale of Goods Act, which says that goods must be 'fit for the purpose for which they are sold'. That basically means that a toaster must toast bread, and a computer monitor must work as a monitor should.