Quick question for anyone who uses Interactive Brokers -- I've been using the interactivebrokers.co.uk website, but am I correct in understanding that this is a US-based organization and therefore my trading account does not need to be disclosed on my FBAR or FATCA declaration?

No, sorry, not correct. (I also have an account with this firm). Interactive Brokers (UK) Ltd is not a US corporation, nor does it have offices in the US. Thus, it is a "foreign" (non-US) account for purposes of the FBAR etc.

I'm still not convinced... From their FAQ, it seems the money is held in a US account (and therefore exempt from FBAR requirements?):

Do I need to complete a W-8 Form as a client of IBUK considering I am a non US resident?

Yes, because the entity holding your client money is Interactive Brokers LLC, in the US.

As a resident of the UK, for my tax purposes solely, can I elect to open an account with IB US directly and bypass opening an account through IBUK?

If you are a UK resident you can only open an account with IBUK. Although, your client money will always be held by Interactive Brokers LLC, in the US.

Only the money is held with IBUS but the account is held with IBUK, so I guess these declarations (no idea what they are) are due on that basis.

The FBAR requirement is addressed to the location of the account, not the subsequent location of the funds. The FBAR (form TDF-90) instructions state:

"the geographical location of the account, not the nationality of the financial entity institution in which the account is found determines whether it is in an account in a foreign country. Report any financial account...located in a foreign country, even if it is held at an affiliate of a United States...financial institution."

Further, as a practical matter, the only cost of reporting the IB(UK) info is measured in minutes, while the cost of arguing with the IRS is substantially more, and the cost of losing that argument is painful.

Does that help ?

For me, where my brokerage account resides determines whether I am required to file for FATCA or not. I agree that if it was just a matter of listing one more account on my FBAR, it's not really worth fighting about.

In any case, I've sent an email to their customer service to see what they say.