That's a good question. I don't have my correspondence with my UK accountant handy, but I believe that this rule of paying CGT for non-UK residents only came into place in April 2015, and one pays UK CGT on gains since April 2015 only. I can show a valuation of the property from before April 2015 which was actually higher than I eventually sold it for in 2016. I should have reported this within 30 days of the sale, apparently. I didn't, and was charged a small penalty for not doing so.
In the US it's another story, of course. I have a ton of depreciation to recapture at 25%, I believe, and CGT probably of 20% + 3.5% "Obamacare" tax.
Of course I might have this all wrong, but that's my understanding.