I used Golden Syrup instead of corn syrup. Americans, if you have not tried this stuff, now is the time! Yum.
It took almost twice as long as the recipe predicted for my mixture to reach 300, but everything really came together very quickly. This is definitely amongst the quickest things we've made.
As for the chocolate layer, I made a choice that I might not suggest repeating: I used a combination of semisweet and milk chocolate chips since I had a little of each that I wanted to use up. It's good this way, but I'd suggest foregoing milk chocolate. It would be better if the chocolate could hold its own against the bold flavor of the toffee. Also, I forgot to buy sea salt flakes, so I had to sprinkle fine crystal sea salt atop. I only used 1/2 teaspoon, because that was enough to cover the chocolate I didn't want to overdo it. This is an okay alternative, but a good flake salt would provide more surprising bursts of saltiness, which is my personal preference.