That's if you give everyone the money, rather than implement it as a negative income tax or do any other kind of means testing to be fair though. There are less than 2m unemployed, so let's call it 5m, and around 20% of the population are over 65, so let's say 10m are retired.

That gets us to 15m, so actually the cost is more like a third of your suggestion (£210bn).

According to this site from the OBR ( https://obr.uk/forecasts-in-depth/brief-guides-and-explainers/an-obr-guide-to-welfare-spending/) we spend about £217bn on welfare, most of which would be repurposed as UBI would replace these.

The hole to fill starts to look a lot smaller, so I can see why people think it's possible.