I think you have received some incorrect advice. While it is true that Delta now prices using married segment logic, that isn't always the case with partners. If you are looking at a wholly partner operated itinerary (which is available only at a low level or not at all), you can add in segments, have the agent reprice and it will come out as an even exchange.
About two weeks ago, I confirmed a business class award on KL using DL miles from AMS to CPT. (65k for the redemption). It took me a few weeks to figure out what where I wanted to stop over in Europe before South Africa. I decided on VCE. So, I called, asked for the VCE-AMS segments to be added to the itinerary. They were sold into the PNR, it was repriced, and for zero additional miles the ticket was reissued. As another data point, searching VCE-CPT displayed no results.
You might be in a different pickle here since you're mixing carriers. Maybe book the SFO-LHR. Then call to ask to add the segment and see if it will reprice at a low level all the way through. If it doesn't, you still have 24 hours to cancel without penalty.