I’m not certain whether or not buying and selling your primary home in the UK while living in the UK is covered by the UK/US tax treaty but maybe someone will come along.
Interesting topic, this. I've been thinking about a related issue recently.
I'm a USC living in my current house for 20+ years; jointly owning with my UKC wife. The value has increased [the house that is; my wife is priceless
] by nearly 3x during that time and were we to sell it the captical gains on my half would exceed the exclusion of $250k.
The link below describes a scenario wherein if you change ownership from joint tenents to tenants in common, the USC can reduce their holding to (or below) the $250k threshold to remove captial gains liability.
https://www.buzzacott.co.uk/insights/stepping-stones-what-americans-need-to-know-about-owning-uk-propertyIt's been in the back of my mind to do something like this, but maybe in stages over a few years so that I don't fall foul of the gift allowance but then don't have any captical gains tax to pay should we ever sell, say to downsize.