Hi
I think the best thing to do is let them know as early as possible that you are "exploring the option of taking this internship" with an explanation of why you are doing it. Everyone in the legal field will understand a younger team member's desire to move up in the world. Your ambition is likely a reason they employ you in the first place! Either they will be okay with it and let you know that its okay, or they will let you know that you won't be allowed to return. In either case, the sooner you tell them, the better.
1. They say this won't be allowed and you can start job hunting now before the vac scheme and give your notice at the appropriate time.
2. They are okay with this and are grateful you have given them so much notice to cover your absence.
Even if they are reluctant at first, an explanation of the significances for this in your career will likely be a compelling reason and they may be more accommodating than you think. In any case, giving your team members ample time to prepare for any absence and plan their workflow is always the best option in my opinion.
