Quick question. The US taxes 85% of US SS and I seem to recall that HMRC treats it in the same way i.e. that 15% of it is tax free and I only need to report to HMRC the taxable 85%. Is this correct or am I just being hopeful? 
The percentage of SS taxed depends on your overall income. If you have only SS as income in the USA, you probably won't pay any income tax on it to the IRS. The info below is lifted from US News and World Report
Individuals with a combined income between $25,000 and $34,000 are taxed on 50% of their Social Security benefit.
If your combined income exceeds $34,000, 85% of your Social Security income could be taxable.
Married couples face tax on 50% of their Social Security benefit if their combined income is between $32,000 and $44,000.
Up to 85% of Social Security income is taxable for married couples with a combined income that exceeds $44,000.
The UK is different When I was in the UK my SS was only taxable in the UK, but my public pension was only taxable in the USA. My SS income (alone) only once passed the point at which any of it was taxable to HMRC - I think I paid +/- 300 pounds' tax that year. It was subject to the personal allowance and then anything over was taxed as income.
