I've calculated the Freedom date of 16th Sep as the date the UK could potentially reach herd immunity through vaccination of 80% of the population.
This 80% figure is based on comments Anthony Fauci made, and is an optimistic assumption that a single dose will give enough immunity.
This date should be taken with a massive pinch of salt, no-one knows yet what will be needed to reach herd immunity. Also I'm a developer and not a epidemiologist. One thing we can say is that the success so far of the vaccination programme in the UK should be a source of great optimism and is a massive credit the the NHS. I built this as a way to celebrate that optimism.
I recommend listening to the How to Vaccinate the World podcast if you want to hear from actual experts about the national and international vaccine rollout. UPDATE: This tool was featured in the listener questions section of this episode of How to Vaccinate the world.