Can any potential lawsuit be enforced on a non US citizen?

[Hypnos]

Hi,  After doing a website for my uncle I also got an extortion letter from Highbee & Associates. They have cited two images which were used on the website. To be honest these were originally put up as demo images and just got overlooked when the site went live. They were taken down upon receipt of the letter but no response was sent to Highbee. I saw it for what it was (an extortion letter) and chose to ignore it. I have since received  second demand letter and my uncle is obviously concerned.   
I have read the various posts and can see the pros and cons of the various options from ignoring completely to using the Defence Letter Program.   
Though my uncles small business is in the US,  I developed the website, the (.com) domian name is registered in my name and I am a UK citizen living in the UK.
My question is:
1) Can any potential lawsuit be enforced on a non US citizen? I am assuming they would have to go through UK system (which is much better at protecting against this sort of blatant extortion).
2) If not is there any way they can go after my uncle (who is a US citizen) and his US based business ?
Many thanks in advance.

[Matthew Chan]

The businesses that have the most to worry about are the ones that have substantial assets like buildings, office, employees, etc.

The businesses that have the least to worry about are the ones that operate from home, have no employees, and assets consisting of a computer, desk, and a chair.  Because the only thing of value is the bank account. But if a bank account were to be closed, there is no meaningful asset.

But that assumes that someone has the wherewithal to spend money and do the work to file a lawsuit.

If the assets are in the U.S., there is more to worry about than if the majority of assets are in the UK.

It is going to take some effort for Higbee to try to file in the UK. But they may not have to depending on how much "business presence" is in the U.S.

[DavidVGoliath]

It's a question of a plaintiff being able to establish facts that the US courts are the appropriate jurisdiction.

You registered the domain name to a physical address in the UK. If the hosting provider was also in the UK then geographically speaking (and absent any further information) the infringement "occurred" in the UK, and any plaintiff would need to bring a claim using the UK court system and adhere to Civil Procedure Rules (see https://www.justice.gov.uk/courts/procedure-rules/civil/rules)

If the website actually displayed a physical or mailing address in the US, then a plaintiff may have grounds to argue that US laws and jurisdiction apply.

As to your own case: was your letter addressed to you in the UK? If the letter is citing US laws to you, then you can go right ahead and ignore it, as a non-US resident cannot be compelled to adhere to US laws. (See https://www.supremecourt.uk/cases/docs/uksc-2010-0015-judgment.pdf)

[Hypnos]

Thank you both for your replies with lots of useful information!
Yes, the letter was addressed to me and no, I am not a business and do not have any employees, premises  or other meaningful assets. As such I will continue to ignore it.
Many thanks again.

[DavidVGoliath]

Just so that everyone is clear: Higbee, a US law firm, quoting US laws, sent a letter to a UK address? Is that correct??