Ok. As a US Citizen you will have ongoing US filing requirement anyway. However as you will be resident of the UK and will perform the services for your employer in the UK, you will have to pay tax on your earning in the UK first of all and then claim a credit for it on your US return when you file them.
Whether you should be self employed (sole trader I assume) or employee you will be taxed the same rate in the UK and US as it will be ordinary income. What will be different is social security tax payments and pension contributions. If you want to continue to pay US FICA and Medicare (as you will be retiring in the US I assume), and continue to contribute to your 401K plan, the best way to do this is to remain an employee of the US company. You can get paid from the US payroll , but instead of Federal withholding, your employer needs to withhold UK taxes and operate shadow payroll in the UK.
If you are not concerned about continuing with the US Social security tax system, then you can either be an employee and get paid from the UK payroll, or become a sole trader and file your tax returns as a self-employed individual in the US and UK. Note that the UK National insurance contributions are different if you an employee or self -employed.
I hope I have answered your questions.
Helen
