She should not be paying tax in the UK under the US/UK tax treaty, which says that certain US government employees are exempt from UK tax - even if the monies are deposited and remitted to the UK. We need to know very gory details to know if this exception works - what division hired her, is US SS tax withheld, where she performs her work, etc., etc. Take this issue up with a tax preparer working in the UK familiar with the US/UK tax treaty.
If the treaty does not apply, then no, your clever clogs idea won't work. There is one possible exception, and that is if she is working on US soil within the UK. Usually, this is defined as the Embassy or a US military base. If she is on US soil, then her workdays are considered offshore. Therefore, paying it offshore and not remitting it should work for up to seven years. I do NOT know the definition of US soil in the UK, therefore I can NOT confirm this theory; hopefully Guya knows the definition better. Otherwise, you'll have to pay a UK person knowledgeable in sovereignty issues to advise you.