@flyoverfred
Very briefly, moving money that you already own from one country to another is not by itself a taxable event. However, you need to be aware of two things. First, banks are required to report transactions more than $10,000. This is mainly to track money laundering and is not a taxable event. However, if you were to structure the transfer in multiple smaller amounts of less than $10,000 to try to avoid the reporting requirement, that can constitute a separate crime. Simply transfer the money by wire or check or whatever is convenient to you and let the banks take care of the reporting paperwork on their end.
Second, if at any time during the year, you hold a foreign bank account with more than $10,000 of deposits, you must report this to the IRS. This is called an FACTA report. You can do this online, it is not included as part of TurboTax but the deadline is the same, April 15 of the following year. Again, this is only a reporting requirement and not a tax requirement.
Separately, how you acquire the money may be a taxable event as discussed by the others. If you are a US taxpayer (meaning that you are a US citizen or green card holder living anywhere in the world, or a non-citizen living in the US more than 183 days) then you are required to report and pay income tax on all your worldwide income. You will use US tax law to calculate your capital gains on your UK properry and pay capital gains tax. You must include the effects of depreciation, which is not some thing I noticed in the prior responses. Your cost basis for the gains calculation is the price you originally paid, plus the cost of improvements, minus depreciation that you took or could have taken (according to US tax law) while the home was a rental. You can also add some, but not all, of the costs of selling to your basis. The allowable adjustments to basis are described on page 8 of publication 523.
https://www.irs.gov/pub/irs-pdf/p523.pdf
If you also pay UK tax on the same capital gains income, you can claim a deduction or credit on your US tax return for the foreign tax as you paid.
