Probably worth signing out and back in and going back to your offers section in manage your account.
The offers come and go so if its not there now, check back in a few days. Definitely worth waiting for Boost to be discounted as they have restarted spamming pre roll (they're not ads but they are) 'trailers' again so it's not worth paying £6 a month. I'm currently paying £2 for boost and wouldn't pay a penny more to be honest.
They might offer you boost discounted or free for a month once you have had one of the other subscriptions for a bit to try to tempt you to spend more.