Also, check the fare basis - 5200 in C might be a D or full-fare C, while the $2600 DL ticket could be a heavily restricted discounted J fare.
I think UA's current Z fare to LHR from the west coast is about $3500, so it IS still more. Others may have more recent info.
.bomb only quotes the same $5200 fare for all dates in June. third party sites are starting to EXCLUDE UAL from biz class searches since UAL is not selling a 100% biz class experience. where else can I find a Z fare on UAL? I've never flown delta biz class but the price seems right.