Below is a copy of the letter I sent to my MP this morning (via writetothem.com). Feel free to re-use and adapt it if you want to send a similar letter.
Update: follow-up letter.
Dear Philip Hammond,
I was extremely concerned to hear of the assassination of two British citizens by the UK armed forces .
The Prime Minister and Defence Secretary justified the attack on the basis of self-defence, which I believe is a reference to Article 51 of the UN Charter . I consider the use of this article to cover action against individuals who are suspected of planning domestic terrorism to be wholly inappropriate. I support those who are calling for a legal challenge against this action.
Please pass on my concerns to the Prime Minister and Defence Secretary, and use your considerable influence in this area to make government policy focus on the use of criminal law to prevent domestic terrorism.
Please work to ensure the government abides by the decision of the UK parliament not to participate in military action in Syria, and thus not to treat this situation as a war in which the UK is participating. As you said on 2nd September 2013, “The House of Commons has ruled out military participation in any such response” (referring to our response to the use of chemical weapons).