I've been a hospital consultant for nearly 15 years. UK medical school, all UK training. I wouldn't want my kids to go into medicine.
The problems as I see it are:
It's a long university course with no special financial support. Yet, it's an intense university course with no real down time and it's difficult to have a part-time job on the side. Graduates are often burnt out before they've even begun. Supporting struggling doctors in Year 1 is common...
It's a highly competitive course that takes the top 5% of academic achievers from school but the salary doesn't compete with other professions taken up by that group. This has been the case for a long time, but there used to be more in the way of non-monetary rewards. Now we have a devalued job that feels less of a profession, and the hourly rate looks bad...
After 5/6 years of medical school, Doctors in training still have a ~10-15 year route to becoming a consultant, longer if they take any maternity leave or work part-time. Those years are really tough, and work out at ridiculously low hourly rates of pay. The system for training doctors, residency, was designed by someone who barely slept and abused cocaine. Google Halsted Cocaine Residency, if you are interested. Yet this remains usual practice in western medicine. A lot is dictated to these "juniors" for those years (actually adults usually 25-35 years old); where they work, where they live, they get rotas with leave that is fixed and dictated, they will be under constant pressure to cover ever present rota gaps, and be expected to undertake the post graduate study required for their chosen specialty while working a variety of antisocial shift patterns. Throughout my time training, seeing people in tears over rota issues was common, people who just wanted to attend a grandparents funeral, their own wedding, see a loved one who was dying, have a honeymoon, take paternity leave when their wife when into labour etc...throughout the NHS there is a computer says no attitude to these normal life events accompanied by the outdated and erroneous idea that the rewards of being a doctor outweigh these losses. Retention, as you can imagine, is poor. Better conditions are to be found elsewhere, people burn out and take time out, many will be put off becoming consultants after this experience, many go part-time even before kids and family life might call for this.
The NHS is a really challenging work environment -most consultants are working at their absolute capacity and more. Training junior staff is ill thought out and relies on the goodwill of each consultant to do "extra". Personally I find that very hard to do -like anyone else I have kids to be home for or to collect by a certain time. There is always a conflict between finishing the pressing work of the day, ignoring the trainee in the room, getting out somewhere close to finishing time and actually doing some training/teaching. I'm sure most people can guess what happens to training in that environment. Trainees will vote with their feet and disappear to NZ/Aus. As an individual I can't change that. I would need another 0.5 of me for the same workload to actually have any meaningful time to train someone. Imagine that number for every consultant working in the NHS right now.
Complaints are really tough. Looking at complaints when they come in, its really rare that a complaint is really about the doctor as an individual. They are almost always about the system. You think the doctor had a bad attitude? Every time I've been tasked with investigating this, I've found a doctor who was over-worked or under-trained/under supported in some way. They got something wrong? Same. You had to wait too long? The whole system, not the doctor. The doctor cancelled your appointment? Lack of flexibility/cover in the system. Apart from rare cases, like Shipman, it's always the system. But when you make a complaint, the individual is made to sweat, not the system. The GMC focuses too much on individuals rather than the corporate and governmental failings...and the real underlying problems are not addressed; the system just loses another doctor.
Overall, I think there is a huge amount of work to be done on retention before simply thinking about expanding the workforce. Or at least alongside.