If you only have one offer then you can't have an insurance choice - depending on your qualifications and the offer conditions for your first choice that might not matter.
Although it's polite to let universities know if you're not interested anymore I would generally caution against burning your bridges too soon. Even if you're certain it is possible that circumstances will change between now and September. UCAS don't ask for you to commit to an offer until early May/June (depending on when you get your last offer)....it is sensible to keep your options open until April at the latest.
Even if you're certain then attending interviews elsewhere could be a good idea. At the worst it will mean you can be more confident that you've made the right decision and give you some more interview experience.