With the new tax rate of 50 per cent for Britain's high earners set to be implemented next April, the Australia international defender is seriously considering a switch to Spain after 14 years in England.
"I am in talks with a club outside of England," Neill told the Australian media.
"That's been the hardest one because of the language barrier. It's a change of what I've been used to.
"A decision will take a lot of courage so that's why I've taken my time."
Atletico's offer of a reported £1.5million-a-year to the 31-year-old is less than what West Ham offered, but Neill will pocket more in Spain due to the country's lower tax rates.