I've had the same experience as hauntedmansion--I've been rejected (or not heard anything from!) jobs that I KNOW I could do...jobs that I've already done in the past. Part of me just tells myself that the job market is competitive, but I will admit to sometimes suspecting that being an American has been a mark against me. I've talked (whined) to DH about it a lot, and he says that my qualifications perhaps aren't translating here...so we're gonna sit down and go over my CV and applications as I do them (I've been doing them by myself up to this point).
As for teaching, I don't think it's the accent that's the problem, but a lack of cultural experience. I am a teacher, but I wouldn't consider teaching in the UK because I don't have the cultural background that my students would have. Maybe I'm paranoid about it, but things are done differently here, and I'm afraid I wouldn't be able to connect with the kids the same way I did in the US (I'm talking about high school aged kids).