On the spot issue of fine for the individual (was £80, but might be £90 now). Beyond that essentially what amplebrew says, it’s at the discretion of the licencing authority / Police / Trading Standards based on the premises previous record and what current measures the company / premises has set up for staff training and enforcing policy and what they intend to do to improve, following failure of a test purchase.
The max fine is £5k to the ?? licence holder? (might be the business owner/company). I suppose the max penalty is removal of the licence either temporarily or permanently. Max penalty to the individual is gross misconduct ie dismissal (most companies have this policy).
I know there is a personal £90 fine and my place of work imposes an instant dismissal from work, but is that as far as it goes?
Yes for the individual, but not for the company and/or license holder.
I don’t think there is any prison time involved for anyone. Certainly there is no way your manager would go to prison for something you did, that’s just someone BSing you.