EC, I had a look at Amazon without logging in to have a look as what happened if I was going to order something.
There's a box which was unticked described as "Yes, I want a free trial with FREE One-Day Delivery on this order." with "
" underneath. So it looks as if you ticked that box and didn't see that.
Perks of Amazon Prime seem to be ad free Prime Music, some of the video streaming is free, and for many items you get free deilvery within one day (in some larger towns and cities you might get same day delivery though that would depend on what time of day you ordered it. Note though that they are likely to use their delivery service rather than Royal Mail and that can be less reliable. The annual renewal is automatic with no notification to you until you see it on your credit card statement. So there are minuses as well as pluses.