US/UK dual taxation on capital gains on sale of property.

I am a dual UK/US national, resident in UK since 1993 and I bought my current home, my principal residence, in 1999. I understand that if I sell my home no UK tax on capital gains is due. However, the US taxes capital gains on sale of one’s principal residence and 25 years of gains on a London home would be well in excess of the personal allowance on the sale. Can you confirm this situation is not covered by the dual taxation treaty and I would be liable for the full amount of US tax even if not taxed on the sale in the UK? Conversely, the US does not tax capital gains on the sale of a rental property if the proceeds are reinvested in another property within a specified period. If I were to vacate my home and let it out for a number of years before selling it, would I then become liable for UK capital gains tax on the sale of the property? Is this situation also not covered by the dual taxation treaty?

Hi

Article 13 of the tax treaty (Uk/USA Double Taxation Agreement - 2002) gives the country in which the property is located the first rights to tax any gains.  
As you property is in the UK and is covered by private residence relief, as your main residence for the period of ownership, no UK tax is payable.  
The UK/USA treaty also give the USA the right to tax any residential property gains as well, using their process for calculating the gain.  
As no capital gains tax is payable in the UK, there is no foreign tax credit to set against any gain arising in the USA. 
If you rent out your UK property, then private residence relief would not cover this period and a capital gain may arise on the disposal.  
We cannot comment on that capital gains tax regulations applied in the USA.

Thank you