No matter where you apply, it is always better idea to apply it from Home country. Because you can show stronger tie with Nepal once you are applying from Nepal and it is easier to convince them that you will return to Nepal after the completion of the study. Where as applying from 3rd country there is always a doubt. It may be difficult to show strong tie with US or home country once you are applying from US.

But then, if you get an offer letter from a reputed university in UK, then more than likely you will be granted a visa provided you are financially strong or have scholarship. However, if you are applying to a JHARE college in UK from  US , it is difficult to get a visa since US is not your native country. I have come across 4-5 student here in UK who did their previous studies in US and they  were studying in a good university in UK.

As you said if you are stuck in US and going nowhere, then there's nothing wrong in giving UK a try.