How do you know the problem is in fact underfunding? The UK seems to spend as much on healthcare as many other first world countries, but with worse results.
Well, let me rephrase. NHS has for a long time been unable to spend money it considered necessary for a sufficient level of ongoing and preventative care.
At the tail end it certainly seems plausible, stories about lack of funds for hiring doctors, and visible queues at A&E have long been a staple of healthcare. What caused that tail-end lack of funds, I'm not sure.