I'm currently reading Who Cooked the Last Supper? by Rosalind Miles (1988: Three Rivers Press, New York). On pages 45-46 she writes the following:
Queenhood was so common among the Celtic Britons that the captured warriors brought in triumph before Claudius in AD 50 totally ignored the Roman emperor and offered their obeisance instead to his empress, Agrippina.
This is given without citation. Never having come across this anecdote before, I went to check Suetonius' Life of Claudius, and Cassius Dio's Roman History in quick succession, and came up with nothing. Putting aside the interpretation, are there any primary sources for this anecdote?
It's in Tacitus, annales XII, section 37, though it doesn't say what your author wants it to say:
ad ea Caesar veniam ipsique et coniugi et fratribus tribuit. atque illi vinclis absoluti Agrippinam quoque, haud procul alio suggestu conspicuam, isdem quibus principem laudibus gratibusque venerati sunt. novum sane et moribus veterum insolitum, feminam signis Romanis praesidere: ipsa semet parti a maioribus suis imperii sociam ferebat.
at these [words] Caesar [Claudius] granted pardon to the man himself [Caractacus] and to his wife and to his brothers. And they, released from their chains, rendered honor to Agrippina also, visible not far away on another raised platform, with the very same praises and thanks with which they had addressed the Emperor. It was indeed strange and not in keeping with the ancients' customs that a woman sit before the Roman standards. The woman herself was suggesting that she was a socius [partner/sharer/associate] of the dominion obtained by her ancestors.