In his book "On Roads - a Hidden History", Pub. Profile Books, Joe Moran states that in the event of a nuclear strike, certain trunk roads were to cordoned off by police to prevent their use by civilian "refugees". Each of the designated routes was named after an animal. BAT was the M1 & M62 London to Hull; HEN was Cardiff to Hemel Hempstead via A48, M4 and A404; FOX was Slough to the North Circular via M4 and A4.
Joe Moran makes no mentions of BUCK, but in the light of the above, first publicised by the investigative journalist Duncan Campbell, it seems likely that the M6 would have been named BUCK.
I have avoided copying verbatim from Moran's book to respect the copyright.
It's a book well worth reading if you are interested in the development of motorways in Britain.