I live in the Uk and my social security should be tax US exempt due to article 17(3) - how do I make Turbotax reflect this ?

Report the income in the social security section of your tax return (to satisfy the IRS), then make an adjustment on the line 21 of the 1040.

Follow these steps:

1. Select Federal
2. Select Income & Expenses
3. Click to Show All
4. Scroll down to the last one Less Common Income
5. Miscellaneous Income, 1099-A, 1099-C
6. select START
7. scroll to the bottom to Other Reportable Income.
8. click START
9. select YES
10. Enter description, US exempt due to Article 17(3)
11. then a negative number. 
12. click Continue
13. click Done

Turbo Tax does not directly support a treaty based position. You can do this by filing Form 8833- Treaty Based Return Position Disclosure.  You can use TurboTax to prepare your tax return to print and mail with Form 8833 attached.  You can access the  Form 8833 HERE.