Someone recently pointed out there's currently a bug with the GUC indicator for D1 cabin. If you search for PS or lower fares on a route, it will only show PS available with GUC option even when there is GUC availability for D1. This is likely what you are encountering. You have to specifically search for D1 fares on a route for it to show correctly as demonstrated below.






GUC upgrades to D1 are based on Z revenue fare class availability now as discussed in this thread -- Using Global Upgrades on Delta (GUCs) post 2/1/2022 Changes
D1 fare classes are J/C/D/I/Z. Since it's a revenue fare class, it pretty straightfoward to figure out dates with availability using "Price Calendar" view on delta.com and Google Flights in US-EU direction simply based on flight prices. One-way Z fares on DTW-LHR route above are $3234 for Tue/Wed travel and $3484 for Thu-Mon travel (these requirements are specified in the Fare Rules found on delta.com). Based on this knowledge, I can tell that DTW-LHR flights on Aug 3 - 5, 9 - 11, 14 - 19, 22, and 28 all currently have D1 availability due to having Z class available. As far as September, only the flight on Sep 29th currently has availability. It's likely most EU routes will have better availability in August than September similar to DTW-LHR route. Finding flights with availability in reverse EU-US direction is a bit more involved since Z fares all require roundtrip bookings from EU direction. However, once you find outbound dates, you can switch to a roundtrip search using one of those dates for outbound and figure out return availability dates based on roundtrip pricing.