BVE they become immune from KM and IJN AAW around 80 for UK TBs base 12. Atleast mine did. Becoming immune from US and UK AAW will take to about level 90 unforunatley but at 80 they only take very minor hits from lots of AAW and you have plenty of time to bomb.
Im pretty sure IJN and MN would be about the same levels as UK with KM, US, SN, and RM needing higher levels