I was looking at a previous question on the subject and it mentioned a form 1120-F for foreign corporations.
Does this mean that foreign corporations who do business in the USA must pay US taxes?
Specifically, a UK-based SAAS company will pay UK corporation tax. Will they be expected to pay taxes in the USA as well if they have any US-based clients? Surely, this can't be correct.
Ye standard disclaimer: IANAL, IANAA
Based on the instructions for the 1120-F the answer to both questions is maybe.
You have to file and you may have to pay. Normally in the US and I would have to assume in the UK the situation is that you can claim a deduction for taxes paid in a foreign country and in addition it is possible that there is an income treaty under which you will qualify for an exemption from paying US taxes.
My suggestion would be to find a good accountant and ask them about it.