I am a retired american citizen marrying a retired British citizen, would like to spend 6 months in the UK and six months in america.

I cannot comment on your ability to remain 6 months in the UK. Your spouse needs to be careful about remaining outside the United States for extended periods of time. A stay outside the U.S. of a year or more creates a presumption of abandonment of lawful permanent residence but lesser stays can be problematic. Your spouse should maintain ties in the United Srates such as a residence, bank account, etc. to show he/she does not intend to abandon his/her status. If you are going to be outside the U.S. For more than 6 months, it is advisable to get a re-entry permit.