What are the caveats to this?

Any other details you can share?

Does this complicate filing taxes on gains at all?

Answer
You may end up paying taxes twice on your forex income. You'll have to pay income tax in the country of your residence (USA). And you'll also have to pay tax on the same forex income in the country where you have your forex account.

There are some agreements between some countries to avoid double taxation like that. But not every country has an agreement like that with USA. Which means that you might have to pay taxes twice. Or else the IRS will be after you.

USA stock brokers that can trade on London Stock Exchange?

I have a stock certificate for HSBA.L traded on London Stock Exchange, does anyone know which US brokers can accept this certificate for deposit and allow me to trade out of this position. What kind of expenses should I expect, e.g. commissions, currency exchange fees etc. Which broker is the least expensive if there are several?

Thanks.

Answer
I had the exact issue a couple of years ago. My regular discount broker (Schwab) could not handle the transaction, but Fidelity handled it easily.

Commission: under 10 bucks
Forex: depends on the size of the transaction, but expect to lose a bit here.

For a one-off transaction, don't try to save $2 on commission. Go to a broker that can get the job done.