US CGT and FIRPTA rules for Brits

[swanie]

I am a UK resident and have owned rented property (residential and commercial) in Florida for some years and pay tax in the US, plus a balancing charge in the UK. I am now about to sell one of the commercial properties and I am expecting to have an immediate 10% withholding tax on the full proceeds which will ultimately be set against a 15% charge on the gain. Is that correct? I have read that non-residents capital gains are taxed ‘as income’ and at 30%, but I assume this is only if you have not operated as a business in the US (rental of property being regarded as a business).

Also, in my circumstances, what US tax rate is applicable for interest received on mortgages granted to US residents and LLC’s? From what I have read about the FIRPTA rules it seems that it may make a difference if there is a ‘kicker’ provision in a mortgage (of US property) when the lender receives both interest and an additional consideration on the asset disposal based on its value?

Any help would be greatly appreciated.