He was a radical, a revolutionary, and a socialist, but George Orwell was hardly a "mainstream communist."
Orwell disliked the Communist Party, and disliked the Soviet Union, for two main reasons:
1. During the Spanish Civil War, Orwell was a foreign fighter who enlisted with a Catalan Marxist militia (called the P.O.U.M.). Orwell saw how the Russian-sponsored Communists betrayed and destroyed the other Left-wing factions in Spain. The experience left him forever embittered against Russian Communism and the Third International. Homage to Catalonia describes Orwell's time in Spain (aside from the politics, if you ever want a good description of what it's like to freeze your ass off in a trench, or to get shot in the neck and survive, read this book!)
2. Orwell detested orthodoxy and hierarchy of all kinds. This shows in his writing in many ways. For example, Orwell hated that some nations had hegemony or imperial power over others. Orwell hated all divisions of wealth and power within society. Orwell hated large bureaucracies such as the BBC, for which he worked during WWII. Orwell even shunned trends and fads in art and literature--and he despised the snobs who rode such trends. The Road to Wigan Pier has a whole chapter devoted to bashing sanctimonious ideologues, whom he saw as inimical to the establishment of a workers' state.
So when Orwell looked at the USSR, or the highly organized and ideologically rigid Communist Parties in most parts of the world, he saw nothing at all that he liked.
But it's not surprising that MI5 kept a dossier on him. That's what the secret police do--indeed that's what they're for. They don't need good reasons to keep a dossier. Their agents often don't really understand what or whom they're spying upon. The apparatus just needs to make a list of people to arrest if the government in power decides to crack down.
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