Eric,
I have no actual personal knowledge of this, but I do know of a family who did "the right thing". They had all the paperwork/jabs etc, on leaving UK; at no time during their total journey did anyone bother, including the ferry. Nobody even "looked" at the 3 cats, despite being clearly visible. The real problem comes when/if you go back, you will need all the paperwork from UK & then into quarantine.