Well first, you need to get rid of Canada. Once that's done, just hop to Greenland from Quebec, conquer Iceland and then conquer the Faroe Islands from Denmark. The US would be in striking distance of Scapa Flow, effectively keeping the Royal Navy focused on the US while the Germans land in England. Having the Faroe Islands also gives the US a strategically valuable spot on which to establish a airbase. Think of Okinawa during the Pacific War in OTL. The problem is, with the invasion plan strung out across different islands, there's nothing stopping a enemy fleet from cutting off supply lines, unless the US kicks naval production into overdrive and starts producing escorts and submarines to protect their ships.
The Shetlands, while it seems strategically viable, is too close to the Scottish coast for comfort and just gives your enemy another supply line to cut.