Is is allowed to use the american passport to go to Uk , while I'm currently living in Romnia and I'm Jordania?

I'm a girl from Jordan- Middle east! and I was born in the united states , so i have the american passport I'm going to Romania on the first of July for an EVS program for six months and I'm doing the visa paper work now because I'm using the Jordanian passport for that i am a Jordanian volunteer . Now i want to travel to the united kingdom using my american passport for 5 days ONLY! is it possible?? entering Romania with my Jordanian passport and leaving it with the american?? and if i left Romania using the Jordanian passport? is is allowed to enter UK using the american??