How much of that "extra" spending on the NHS is just cost of living rises? By my reckoning, about £21bn. Which means even if Boris hits that target (and once Brexit fucks up the economy, things like that will fly out of the window so fast your head will spin) you are only looking at an extra £11bn. Which is only ~£200m a week. So the best Boris can promise is that 8 years after the bus, it will only be about 40% a lie.
That particular unicorn may not be dead yet, but it's looking pretty fucking sickly.