They came within 6 weeks of bringing famine to the UK. Had the RN not reluctantly changed to organising convoys they could well have succeeded.
Six weeks supply is different than famine in 6 weeks. It means that if no more ships are unloaded and no corrective action is taken, then the warehouse are empty is six weeks. I guess we need to get to the numbers.
Women probably need 3,000 calories per day, 4,000 for men. They are doing manual labor. So lets take men only for easy numbers. The UK can generate 60% of needed food, or 2400 calories per day. But this 2400 calories are not evenly spread in the year. The UK food production in winter is low (milk?), so it is probably above 3500 in summer, probably under 1000 in winter. And you don't quickly starve to death on 2400 calories out 4000 needed. You lose about a pound every 3 days, and then you will start to work slower.
When I say excess deaths, I don't mean people who look like they are in concentration camps. It is a lot of non-fatal illnesses become fatal with a 1600 calorie deficit. It is why people spend so much time arguing over how many Germans died in the blockade.
But against a population that has already lost weight and the tendency to mismanage a bit, I do expect the UK to leave the war. If the reaction time is a little slow, we can easily have a few months in the winter where food goes below 1500 calories per day for many people.
So then if we are writing a good ATL, we get into a bunch of arguments that sound like we are on some Soviet style planning commission. And that is why we see different things. Some will see very good projections and rapid reactions and the UK quickly dropping to a sustainable level. I see slower reactions as likely and a much tougher winter. Then we deal with secondary impacts such as "How much does food production go down if half of tractors are idle?" "How much slower does a farm worker work who has a 1000 calorie deficit?" etc. etc.