The RCTS history of the BR Standards (Part 1) contains information about Britannia names, but I can find no reference to a possible John F Kennedy name. As for why 70047 received no name - they probably never got round to it as steam was being phased out. Initially none of 70043 to 70054 had names.
70050 - 70054 received "Firth" names when sent to Polmadie soon after they were built. 70049 only got its "Solway Firth" name after transfer to Scotland in 1960. 70043 -46/48 got their names in 1957/58. The RCTS book notes that the name on 70048 was "planned to be transferred onto a diesel loco. in due course. "
The RCTS book also mentions that patterns for Britannia nameplates were discovered for the names of three loco. engineers, but the names were never used (Sir John Aspinall, Samuel Johnson & John Ramsbottom, from the L&YR, Midland R. & LNWR, respectively)
Bevan Price