You can stay on the US payroll, but they should still operate PAYE if they have a permanent establishment in the UK, under the UK PAYE definition of PE, not the US one or the treaty one. They should take professional advice before going down this road - it is they who are liable for the PAYE (and the penalties and fines for not withholding), not you. I recommend they take this advice from a qualified UK firm specializing in multi-national tax issues, like the Fry Group
www.thefrygroup.co.uk.
If they have to do PAYE, they can do a modified PAYE to take into account various deductions. If your secondment is for two years or less, you get a massive number of tax adjustments, which will be reflected on your modified PAYE. The tax advisor can provide all this information to your employer. Further, your US tax withholdings can be reduced by filing Form 673 with your employer.
http://www.irs.gov/pub/irs-pdf/f673.pdfIf, after taking the professional advice, it is determined that there is no PAYE requirement, then just file the 673 and pay your UK tax on a self assessment form.
Your employer should also pay for a US/UK tax preparation as it's confusing in the year of your move.