What Hotels are there around National Portrait Gallery London?

We're traveling to England for the first time and we need to decide where to stay. We know that we have to be very close National Portrait Gallery in London because we're attending to an event. However, I've never been to England and have even no idea what that area is called. We need a middle class hotel, not a super cheep one necessarily but not a hilton either. It is harder for me to run a search on hotel search sites, because they show me a wide range of hotels in neighborhoods that may possible be in a very far part of London. Help me please! You can tell me where to search, such as area names and stuff like that.