Where did you buy it from? If it was from a provider (EE, O2 etc) or online retailer that provides phone+contract package then the handset will be running a modified operating system for/from that provider and you won't be eligible to participate in Beta. You'll have to wait until both the official release hits and the service provider that handset is on releases their 'tweaked' version of the OS...which could be days or weeks after the official release.
Beta is only open to handsets that are true factory unlocked and non carrier; so devices supplied and bought direct from Samsung UK, and the likes of Currys / Argos who source & supply direct from manufacturer.