Most of the answers seem UA-centric, so here's a view from another airline.

A little over a year ago, I wound up having to fly on American trans-Atlantic. I'm not an elite with AA (yet -- should make Million Miler soon, mostly from non-flight miles, and that will make me Lifetime Gold), but on the day of departure I asked at LHR whether I could upgrade using miles, and was told "sure". Needless to say, I did so.

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MM=> UA 100.7%, AA 99%