You're missing the point, which is that the number of people who die from cancer that would otherwise have been treatable except for their ability to afford it is actually quite low. And the US does much, much better at treating cancer overall. Which is why the overall cancer survival rates in the US are drastically higher, even though a small number of people may not be able to afford treatment..

(You're also assuming that people are able to receive cancer treatment in countries like the UK, which is actually not a foregone conclusion. Assuming it's even diagnosed properly - the UK in particular is really notorious for cancer misdiagnoses - they may actually not be eligible for having their treatment covered under the NHS, even though their cancer may otherwise medically be treatable, either in other countries or if they can self-fund it in the UK).