I saw it referenced in account management docs, trading uk cfd and metal docs etc. I have to admit at first i did not know either because the default account you use will be the non-F account. The F account is only always talked about when you want to specifically trade UK stuff in the UK.
The way i found out is when i came across multiple references that IB UK accounts are indeed fdic etc insured + cash sweep capable for much larger sums than the measly 85k gbp. When i started digging about that i found multiple references in the IB UK docs itself as well as on the net.
As for the IB UK ToS well basically the whole document is about how you are really dealing with IB LLC. As in regard to deposits this is probably the corresponding section:
"B.United Kingdom Client Money and Custody Rules:i.IB LLC is located outside of the United Kingdom and IB LLC may deposit and hold money,securities, and other assets (including, but not limited to, collateral and safe custodyinvestments) on behalf of Customer in accounts that are located outside of the UnitedKingdom. The legal, regulatory, and settlement regime applicable to IB LLC and to theentities in which Customer's money, securities and other assets will be held will be differentfrom that of the United Kingdom (i.e., any client money and custody Rules promulgated bythe FCA will not apply). Among other things, different practices for the separate identificationof Customer's money, securities, and assets may apply and, in the event of a default of IBLLC or the entity in which Customer's money, securities and other assets are held,Customer's money, securities and assets may be treated differently from the position thatwould apply if the money, securities or assets were held in the United Kingdom.ii.To the extent that IB LLC holds Customer money, securities, or other assets, IB LLC will holdsuch money, securities, and assets in accordance with the rules of the U.S. Securities andExchange Commission ("SEC") and the U.S. Commodity Futures Trading Commission("CFTC"), as applicable."
https://gdcdyn.interactivebrokers.com/Universal/servlet/Registration_v2.formSampleView?formdb=3207