Your sale in 2000 was taxable under the rules in effect in 2000, meaning your capital gain was taxable, subject to a personal exclusion if you were selling your main home where you lived more than 2 years. What you did with the money after is irrelevant.
Similarly, the sale of property outside the US is reported on your US tax return the same as if it was property in the US. It doesn't matter where the money came from or what you do with the money later. The sale is taxed on its own. If this is your main home that you owned and lived in as your main home for at least 2 years, you can exclude the first $250,000 of capital gains (or $500,000 if married filing jointly) and the rest is taxed as long term capital gains. When you calculate the gain, report the purchase price in US$ using the conversion rate in effect on the date you purchased the property, and the selling price in US$ using the conversion rate in effect on the date of sale.
If the UK also taxes the sale, the US should allow you an offsetting credit or deduction, I believe.
