The money is apparently coming from all that "economic growth" the Tories think we'll get after Brexit. https://www.theneweuropean.co.uk/top-stories/boris-johnson-again-pledges-to-deliver-350-million-a-week-for-the-nhs-1-6197953
Poll finds 74% Britons think Brexit deal is unlikely before October 31st https://www.theneweuropean.co.uk/top-stories/poll-finds-74-britons-think-brexit-deal-is-unlikey-before-october-31st-1-6194426
UK economic outlook 'very murky indeed' thanks to looming no-deal Brexit, says think tank https://www.theneweuropean.co.uk/top-stories/niesr-report-on-brexit-and-the-economy-1-6174155