Short answer: Yes (i.e., the fact that one party is outside the US does not make an otherwise enforceable agreement unenforceable).
Slightly longer answer: Yes, but, unless you are a multinational business, the process of enforcing an agreement against someone in a foreign country is so expensive and so uncertain of outcome that enforcement may not be practical.
This information does not constitute legal advice and does not establish an attorney-client relationship.
This is a common question for businesses that hire across borders. In fact many (most?) businesses are using independent contractor relationships across borders.
This relationship can be great... or a big headache. If it's truly an independent contractor relationship under the foreign laws, then the employer gets the talent without the employer responsibilities. If foreign laws create an employer-employee relationship in fact, then the employer is on the hook for payroll withholding, employment law issues, corporate tax liability through a permanent establishment, and corporate registration issues: a nightmare!
All that said, the UK and Canada are relatively easy to troubleshoot. I've helped businesses with independent contractors and employees in both jurisdictions.
In short, you'll want to determine the factual--an in turn legal--nature of the relationship. This will let you know your risk as an employer. I can walk you through that analysis in a short phone call.
Now, if you are asking whether the terms of the agreement are enforceable, it generally is in Canada and the UK--especially if you have a well written agreement that complies with local laws in Canada and the UK. You'll want to include some practical terms in contract and thoughtfully consider how you structure the relationship from a non-legal perspective.
If you are interested in chatting further, please feel free to email me (firstname.lastname@example.org) or call me (+1 425 351 9455) directly. While you might be able to figure out this information on your own, it would not hurt to go through a free consultation to point you in the right direction.
Total Mobility Law is an international law firm that lets companies do global business with the knowledge and confidence they need to comply in any country. Our answers on this site do not constitute legal advice, nor do they establish an attorney-client relationship. The only thing that can do that is a signed Engagement Letter and Fee Agreement, which you can get by contacting us through www.totalmobilitylaw.com.