I will bet my house that sometime you were cautioned. PACE dictates that the police have to as soon as practicable. That means, it will not necessiarily been given at the time by the arresting officer.
The custody sergeant HAS to give it aswell and when you accepted the ticket, you would have been given a slightly different caution but on the same gist.
The reason you were not interviewed is because Police officers (not doctors, not witnesses and not you) are deemed "expert witnesses" on regards of you drunkenness. So what is the point of an interview. It really is their word (testimony) against your proven innocents at being sober. The only defence if you decide to go to court will be
1) You were under medication (which will need full medical testimony)
2) You can prove your drink was spiked (A Mickey Finn).
Best advice, pay the ticket. And take it as a very stern warning. And seriously, I would write to the officers involved and say.............................
"I now realise that you could have taken me to court for what I did and i thank you for using your discretion and issuing me with an FPN" blah blah blah.