Chazal (Avodah Zarah 35b) were concerned that some non-Kosher milk may get mixed into Kosher milk, and so instituted that milk production must be supervised by a Jew who must ensure that any parts or containers used don’t come into contact with non-Kosher milk.
R’ Moshe Feinstein (Igros Moshe YD 1:47-49, 2:31, 35, YD 4:5) wrote that so long as the milk production is regulated by government standards, it is considered supervised. Such milk is referred to as chalav stam. Initially R’ Moshe wrote that while a baal nefesh should refrain from chalav stam, one mustn’t criticize others who drink it. In his later Teshuvos, (perhaps as chalav yisroel became more prevalent in the US), he wrote that one should only rely on this hetter under extenuating circumstances.
While many in the US follow R’ Moshe’s (earlier) psak, in the UK (and Eretz Yisroel), most follow R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 1:138, 2:21) who didn’t rely on this hetter. Certainly, in a country where the dairy farms aren’t strictly regulated, one would not be allowed chalav stam.R’ Tzvi Pesach Frank (Har Tzvi YD 103-104) held that the issur of chalav akkum never included powdered milk. Thus, there are some who are makpid not to drink chalav stam, though will still eat a Mars Bar. Most Kashrus authorities follow the Chazon Ish (YD 41:4) who doesn’t differentiate between milk and powdered milk (See Shearim Metzuyanim Behalacha 38:12 and Teshuvos Vehanhagos 2:373).