ANO Rule 8. Avoiding Aerial Collisions.
(3) Subject to sub-paragraph (7), aircraft shall not fly in formation unless the commanders of the aircraft have agreed to do so.
No-one with half a brain will do it without proper training and proper briefing, although the only legal requirement is for all commanders to agree.
Here somewhere.... 
Edit: Besides, I'd hardly refer to 1nm separation as "formation". More like "in the vicinity".