Depends on the product. a food delivery app or one that deals with taxes in the UK, maybe not, but a SaaS platform to serve AWS users, I'd imagine users in Australia could make just as much use of that as in the UK, and Stripe makes it easy to accept payments from there.
Unless you're pulling an unscrupulous rando of fiver, it's better to presume that your employee is a professional and won't do that, rather than burn a large amount of resources protecting your source code. What could I actually do as a programmer with, say, a copy of the Photoshop source code?
if your SaaS app is, say, some intricate detail in the UK VAT system, how much could the code even be worth to someone in Australia? even if they did steal your code, do you have the energy to make an Australian subsidiary and pursue that? the opportunity cost would be high.
for that matter, if you're going to go down the route of not trusting your employees just to make sure no one wants to work for you even more, are you gonna drug test them as well? pay them what they're worth and treat them well, and they won't try and steal from you.
a standard employment contract should cover "don't steal our source code" without going to the lengths of being an NDA. that's not to say NDAs aren't used in the industry though. For what it’s worth, noncompete aren’t enforceable in California and that hasn’t seemed to stop its tech industry one bit.