What is my tax liability to the IRS and HMRC if I earn affiliate commissions from organizations such as Clickbank? I am a UK citizen, but statistically (I have heard) most of the earnings come from the US. Do I need a trading presence in the US, or just in the UK?

If you have NO physical presence in the U.S. then you have NO tax liability at all to the U.S. This assumes the payments from Clickbank are commissions for services rendered or products sold.

If they are royalties, then some U.S. tax MAY be due.

As for the U.K. tax liability, you will need to consult a UK tax expert on that. This forum only addresses U.S. tax issues.