There remain plenty of Labour MPs who voted in favour of the Iraq war are there not? Only Diane Abbot can claim to have been anti, and even she wasn’t so principled to have considered it worth resigning over (unlike Robin Cook).
Actually there were 139 Labour MPs who opposed the Iraq War compared to 15 Tory MPs who were opposed.
And Robin Cook resigned as Foreign Secretary, what exactly did you expect Diane Abbot to resign from?
I personally saw Diane Abbot on an anti-Iraq War demonstration milling unaccompanied among the tens/hundreds of thousands other protesters, she was clearly there on a matter of principle.