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Both. Do both. Make it easier for them to address the issue than ignore you. Depending on which side of the aisle your MP is on, focus your letter on either “those evil <other side> are doing thus terrible thing, I know you’re bold enough to stand up to them.” or “this policy seems to have the following problems, and it’s leaving you open to attack from <other side>. It’d be a shame if you lost your position over it.”
Matching communication styles is the key here. If there’s a general chatty vibe to the team and you’re the only one not engaging then you’re the odd one out and that will invite comment.
If your team are chatty, you don’t need to go all out at the same level they are, but showing some willing and chatting at least sometimes will usually be enough to ensure harmony. Knowing how to disengage without causing offence or annoyance is also an important skill.