https://www.unblock-us.com/
It costs $4.99 per month, or $49.90 a year, but is IMO well worth it if you want to watch more than just Netflix UK.
The region switcher on the website means that if you want to go back to Netflix UK from Netflix US (or vice versa), you don't need to touch the DNS settings on the Xbox (or whatever device you use them on) - you just change regions on the website. It also opens it up to other Netflix regions such as Canada and various ones in Europe etc.
Yes, it costs a small amount each month, but it's an established company with a good rep. There's no way that I'd use some random anonymous DNS settings found via Googling or whatever.