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
The British economy has been hurt by “intensifying Brexit-related uncertainties on business investment and weaker global growth.” https://nyti.ms/2SUMWwQ
