When DL shifted the LHR operation to entirely 764s in 2009 it was because the 764 was the only fleet type at the time that had flat bed seats on every frame. Even the 777-200ERs didn't get the same herringbone seats as the 777-200LRs until 2011, 763s began later in 2012, and the A330 didn't even begin until early-mid 2013. Also remember in 2009 DL/NW was still operating at LGW, IIRC NW operated ATL on the 333 and DL ran CVG on the 763.

It wouldn't surprise me if DL shifts things around at LHR and substitutes other fleet types, but I doubt we will see large-scale A350 action there, if at all. I also wonder if DL is testing this like they did Economy Comfort years ago. EC was initially limited to international and later expanded to domestic. Perhaps their research suggests target market only has a sufficient willingness to pay on ultra long-haul flying.