The way it has been handled is appalling but step back and ask yourself what you want to achieve, because you are making a rod for your own back.
Now by my calculations if we assume the course starts in, say, 3 weeks' time then you will not have completed it until the last week in April, so even if you started using it straight away then the NHS would only 'benefit' from your learning for 5 weeks before you go on maternity leave... it would make more logical sense to get agreement to go on such a course when you return.
Your boss may be treating you in a way that is upsetting, but coming down on him like a ton of bricks won't change his attitude. And even if it IS discrimination, I think you should choose the battles that you fight and I don't think this is the battle you want to make a stand on. I would simply mark it down and see if anything else happens.
The reason I say this (and I speak as someone who has made a stand on the sex discrim front and had to leave a company) is because even though the law is on your side, in this situation it is an uphill battle. Yes you should get the same pro-rated entitlements when you go part time but in the eyes of your employers they are not usually wholehearted supporters of part time working - they do it because they have to, and often even HR expect recipients to be grateful and to view it as a 'perk' - when you then demand entitlements on top you could tip the balance and be seen as 'grabbing' what you can, and this can really count against you in an employers' eyes. It is not fair but that is how it operates. The less flexible and more demanding a person is, the less willing the employer is to invest in that person, and there is a real risk in the circumstances that you describe that you are making yourself look less good in their eyes.
I am a part time mum and I completed my thesis on flexible working and I assure you I am 100% in favour of part time working but even I would hesitate to fund something that might only be used for 5 weeks. I think you should step back and try to appreciate it from the employers point of view.
FWIW if you DID complain I imagine you may well get your line manager to back down, but he will not thank you for it. Courses DO cost money, they DO mean that shifts and rotas need to be rejigged and for a 5 week benefit it just doesn't stack up. much much better to buckle down and show everyone above you how valuable a team member you are, so that you get them to start being 'blind' to your part-time/PG status. When you are gone on ML they will appreciate you more and you can build on that after you return.
And if the line mgr really IS discrimantory (and I don't think the evidence here proves it TBH) then you will find more and more things not adding up and then you will get much more evidence to be able to build up a case to complain about, and you will then be attacking it from a position of strength.