As it happens this is a subject I know something about. In recent years the quality of medical staff coming from abroad was excellent. Now it's less so in some cases. Hospital Trusts increasingly use Agencies to recruit staff particularly from Africa, India and the Phillipines. Before an applicant gets accepted they have to sit an English exam. Those coming from most Malaysian countries are generally excellent. The problem in some cases is that when the new recruits arrive at their allotted NHS Trust many can't speak English suggesting that others are taking the exam for them. The upshot is that Hospital Trusts have to send the new recruits for English lessons. The same goes for technical competence where the new staff require a lot of supervision, have little idea how to handle patients leading to the suspicion their qualifications maybe as suspect as their English tests. I think this issue was touched upon very subtly by an earlier poster.
I emphasis that this is a relatively recent problem as up to fairly recent times the calibre of recruits was good and evidently the NHS couldn't survive without them.
The massive push to recruit staff is the issue here. The present government's desire to recruit 50000 nurses means that their recruitment standards have lowered. Had the government trained its own staff this issue would not have arisen. Their decision to cease bursaries a couple of years back hasn't helped and they have now reinstated them but too late.
However, as a general point and notwithstanding what is said above, it doesn't seem morally right for the UK to poach staff from countries whose need for medical staff is probably greater than ours.