it would seem that Milliband senior didn't much like the establishment, and if you think that was a rather large part of people's lives once upon a time, Royalty, Church, and the State, so if he rubbished them when a young man, and indeed as he got older, irrespective if he served in the war then some would see that as not liking Britain very much,.
his Marxist teachings, working at the LSE would rather suggest he was a very left thinking person, and at the time that was akin to anarchy, not so much now. The establishment is still there but is fast diminishing, the church and the royals are no longer major players in every day life in Britain,
so his views at the time and later may be construed as rather anti British.
not everyone who fought in both wars wanted to be there, many i am pretty sure didn't. He volunteered, perhaps it was his way of attacking those who had his homeland, same as the Polish airmen, and many more besides.