Not exactly the same flights on the outbound, but gives you what you want (native UA flight number on transatlantic). Inbound is same

This is bookwithmatrix (selecting the Priceline option). Priceline interface doesn't tell you the native numbers until you select both legs, you could probably find your exact match after some trial and error. Haven't tried ticketing of course, will leave it up to you (I charge 1% search commission fee)