Hi,
The double taxation agreement between the UK and the USA, allows the UK to tax income / gains from an 'immoveable assets' arising in the UK, such as property income. This is done by completing a Self Assessment Tax Return, to declare only that income which is taxable in the UK. You do not declare your non UK income on the tax return, so should not complete the foreign section at all (SA106).
The paper tax return just to declare UK property incone, would comprise of SA100 (tax return), SA105 (property) and SA109 (Residence). Paper versions of the tax return and supplementary pages are available at:
Self Assessment tax return forms
Thank you.