I have received a response from SIPC, confirming littleadv's answer:
For a brief background, the protections available under the Securities
Investor Protection Act ("SIPA"), are only available in the context of
a liquidation proceeding of a SIPC member broker-dealer and relate to
the "custody" of securities and related cash at the SIPC member
broker-dealer. Thus, if a SIPC member broker-dealer were to fail at a
time when a customer had securities and/or cash in the custody of the
SIPC member broker-dealer, in most instances it would be SIPC's
obligation to restore those securities and cash to the customer,
within statutory limits. That does not mean, however, that the
customer would necessarily receive the original value of his or her
purchase. Rather, the customer receives the security itself and/or the
value of the customer's account as of the day that the liquidation
commenced. SIPC does not protect against the decline in value of any
security. In a liquidation proceeding under the SIPA, SIPC may advance
up to $500,000 per customer (including a $250,000 limit on cash in the
account).
Please note that this protection only applies to the extent that you
entrust cash or securities to a U.S. SIPC member. Foreign broker
dealer subsidiaries are not SIPC members. However, to the extent that
any assets, including foreign securities, are being held by the U.S.
broker dealer, the assets are protected by SIPC. Stocks listed on the
LSE are protected by SIPC to the extent they are held with a SIPC
member broker dealer, up to the statutory limit of $500,000 per
customer.
As I mentioned in the comments, in the case of IB, indeed they have a foreign subsidiary, which is why SIPC does not cover it (rather they are insured by Lloyds of London for such cases).