Upscaling a huge organisation is a difficult process. To build a pipeline of new doctors and nurses, you need to be training them - or, if you choose to rely on immigration, be offering a far better deal than other countries. You'd not be building new hospitals, you'd be building new medical schools, trying to entice people into the professions. It's not particularly easy - and a lot of that is more than just throwing money at the problem.
In any case, where's the line drawn? We currently spend more on the NHS, even adjusting for inflation, than ever before in history. Where is the level at which we say 'oh, that's it - it's fine'. Because I don't think there is one.