I am a retired UK citizen travelling to Moscow.
In UK I am able to buy museum tickets at the lower price (senior citizen price).
Will I be able to buy museum tickets at the lower price or get a senior discount in Russia/Moscow?
No, you won’t be able to buy discounted tickets. This is only available if you are also a citizen of Russia. If you are a citizen of Russia, then most museums will offer discounts with relevant documentation (e.g. passport and retirement card).