I would suggest that the Myanmar situation is closer to genocide as it is understood, it is taking a specific ethnic group out of your own country and doing so slowly the way that Germany did it in the 1930s rather than quickly as happened in Rwanda. What is happening in Israel may meet the definitions for war crimes (in specific instances) and Hamas certainly do but I don't think the Israeli action meets the definition of a genocide. In short, I think the UK and US position (whilst morally dubious) is technically correct. If not entirely correct, then correct enough that wiggle room will work.