"The police officer was only doing his duty, so he is in the right."
That is total bullsiht.
We have to treat this horrible pernicious disease with respect and act sensibly, but that doesn't mean we have to abdicate all common sense. And this was an abdication of common sense.
I'd have pleaded not guilty anyway, "criminal damage" must actually cause damage, chalk marks will be gone as soon as it rains and I think there is a precedent for that not being damage.
Section 1(1) Criminal Damage Act 1971 - A person who without lawful excuse destroys or damages any property belonging to another, intending to destroy or damage any such property, or being reckless as to whether any such property would be destroyed or damaged, shall be guilty of an offence.