If you have valid US visa you don't need UK transit visa. I went to Nepal without transit visa in May and my two friends also came via Hethrow this August without transit visa. But if you want to visit your cousin, you have to go out of immigration and you will need visa (transit visa won't work for you). I am also going to London on Dec. 19, but I already have my visa. I applied for it when I was in Nepal. Check their website. The website has definition of all visa types. Hope this helps.