I don't believe any airline allows for free upgrades on European flights. Per United:
Complimentary Premier Upgrades are available on most United- and United Express-operated flights in the following regions:
Continental U.S.
Hawaii
Alaska
Canada
Mexico
Central America
Caribbean
Oceania
Originally Posted by
jr1202sr
I just flew on United from London to DC and as a silver didn't get upgraded with about 5 Biz Class seats empty on the plane. I even confirmed with the FA that indeed the seats were never allocated and were empty. I was in Economy Comfort (paid for it).
Just a reference point for what Silver gets you vs Marriott Gold.