'Warranties' and 'guarantees' are entirely separate from your statutory rights. They are 'gifts' from manufacturers (or, less commonly, from retailers). As such, they can be as liberal or as restricted as the giver wishes them to be.
For example, a tent company could give you a guarantee which only came into effect when you returned their defective product back to their factory in China in person, between 11pm and midnight on a Thursday with an even-numbered date, while singing the Mongolian national anthem backwards in the nude. Such a guarantee would be perfectly legal but it wouldn't take away your statutory rights (which are with the retailer, not the manufacturer).
Most guarantees/warranties are provided by manufacturers, rather than by retailers, and it's standard practice for the guarantee/warranty on a replacement product to start from the original purchase date, rather than from the date of replacement.
Chris