Likely JetBlue.

If you are willing to connect from another European city, that would open up more opportunities - e.g. EI (Aer Lingus), SK (SAS). I find both do their lie-flat to be good, along with superior inflight services when compared to UA, which is not that difficult.

From DEN, aside from UA nonstop to London, you have quite a bit of one-stop options.