Took me a bit of dicking around, but you have to change the DNS settings on your router to make the website think you're in the US. You can find free DNS servers to point to, but Netflix block them regularly (apparently), so there are places that you can pay for access to DNS servers which are guaranteed to work. I signed up with Unblock-US but others are available. Then, you just go to the NetFlix app (or website) and the US selection comes up. You'll know the difference as the recommended titles will be full of stuff that doesn't come up in the UK.
It's one of those loopholes that NetFlix currently don't give a shit about. They have licences to show certain titles in certain regions, but no doubt they tell the licencees that they've taken steps to restrict access from other regions (which they have), and that'll continue until the right holders start kicking up a big stink about it. In other words, they don't ban anyone for doing this