Your US license was only valid for one year in the UK, so unfortunately you will need to get a UK license before doing anything. You'll be a learner status, meaning you'll have to have a full UK licensed driver (I believe they have to have held the license for a minimum of three years) for you to drive prior to receiving your full UK license.
However, from what I understand, once you have your full UK license, you can then apply for a provisional motorcycle license. As long as the engine is below a certain size, you can use "L" plates on the motorcycle for as long as you like (meaning you never need to take the full motorcycle test).
Unfortunately as your US license is not considered valid in the UK as more than a year has passed since you moved here, you will be uninsurable until you start the UK license process.