Here is what I said to Mayor Sadiq Khan via his website. You may want to write your own.
Dear Sir,
Please accept my humble condolences and healing prayers for those injured and killed in the recent terror attack; I hope this finds you otherwise well. I realize you are very busy, but I just wanted to take a moment to apologize for the actions of “our” president. Not all of us voted for him; not all of us share his boorish opinions or worldview. Many of my countrymen/women are otherwise fine people, and many of them are trying their hardest to mitigate the damage he is doing to our country and the world. I can only hope you don’t judge us all by the actions of one man, even if he claims to be our elected representative.
Be well, and thank you for your time,
Neal Ross Attinson