UK-based clients trading through a US-based, US broker are not afforded the same protection as US-based clients trading through the same broker.
UK-based clients trading through a UK office of a US broker are protected by FSA rules re compensation, DEPENDING on the clients' classification, ie Non-customers (marker counterparties); Customers - Non-private, ordinary business investors; Private - small businesses (trade customers), Private - individuals; Private - expert. And so on.
On a very simple level only Private Customers (individuals) are protected. However, to avoid the need to segregate clients' funds from the firms, some firms may designate private customers as 'experts', whether the clients like it or not. What this means is, if a firm is short of funds, it can use the clients without recourse. Further, they'll get nothing from the Investors Compenastion Scheme if the firm goes down.
Barings and Enron were well capitalised