This is usually only done with John Lewis, but you do not have to buy from John Lewis to get the 5 year guarantee.
I recently bought an AEG dryer from an online retailer that was much cheaper than the John Lewis price. I then used the 5 year warranty registration form on the John Lewis site. Worked a treat.
There is nothing in the terms and conditions stating that you must buy from John Lewis to claim the free 5 year warranty!