The first thing to make clear is that employment discrimination, per se, is not illegal. It's only specific types of discrimination which are illegal.
For example, I used to work in a school where the headteacher's policy was that he'd never promote anyone internally. He's always bring someone in from outside, even if the best teacher in the country was already working on his staff. That was his publicly stated policy (because he believed that no teacher should remain in the same school for more than a few years). It was discriminatory but perfectly legal.
Any employer can choose whether to advertise a job internally, externally or not at all (and simply give the job to whoever he likes). There are a few restrictions on his actions in the case of a limited company (where he must put the shareholder's interests first) or where public money is involved, as in the case of the NHS, where it's the public purse which should come first (ahead of the interests of existing employees). It would seem that the employer may well be seeking to meet a duty to get maximum 'value for money'.
The actions are certainly discriminatory but almost certainly not illegal. The reason I find it necessary to add a note of uncertainty, through my use of 'almost', is that there's a possibility that this practice might be regarded as indirect age discrimination which, of course, is illegal. (i.e. by making posts available to newly qualified personnel, they're effectively only offering most of the posts to younger health professionals).
To check whether this amounts to age discrimination, phone the Equality & Human Rights Commission helpline on 0845 604 6610.
Chris