There is no easy way into orthodontics. The UK system is also equally competitive as it is here. It's a little insensitive to think that the foreign grads are being treated with kid gloves compared to the local US grads. The admissions policy works the same for both groups, and without any bias.
Bottom line is, if you think the foreign grads got in just because they were foreign grad, then you are sadly mistaken. It is actually much harder to apply as a non-local applicant simply because no-one knows anything about you.
So if you applied to the UK orthodontic programs as an international student, you would only be considered if you satisfied their requirements such as MFDS, research, house surgeon posts, vocational training etc. Oh, and you also need to show them that you have enough cash asset to apply for your student visa.
PS. PhD in orthdontics? That's okay if you want to stay in the academics. The PhD in the UK is very different from the US PhD system. I don't know the details. But there is no PhD in "orthodontics". You would have to read basic or clinical biomedical science subject and perform research in the field of orthodontic science, may take you 5 or 6 years. But you won't necessarily do clinical ortho training and obtain your specialist status. You will be a research scientist, not a specialist clinician, should you do a PhD in the UK. Now, in the US, there are some programs designed to let you train in both PhD and ortho specialty. But across the Pond, none (last time I checked was last week!).