CGT on USA property sale for UK citizen

I recently sold my holiday home in Florida and found that 15% of the sale price was held back under a scheme called FIRPTA (Foreign Investment in Real Property Tax Act). My question is: do I pay my CGT to IRS or to HMRC (I believe that there is a Double Taxation Agreement [DTA] to avoid having to pay to both). If it is paid to the IRS then I would assume that they take it from the 15% that has been held back. If it is paid to HMRC then how do I get my 15% returned to me from USA and how long does that process take? Typically, what forms do I need to complete for HMRC?

Hi,
 

UK residents have to declare foreign Capital Gains to HMRC, so you should enter the relevant details in the Capital Gains pages (SA108) of your Self Assessment tax return. Foreign Tax Credit Relief can be claimed (in the Foreign pages/SA106) on the actual amount of US tax you eventually pay on the transaction.    See Link : Relief for Foreign Tax Paid 2021 (HS263)  . Thank you.