Actually, from what I have read, it's pretty well acknowledged in Britain that Philby was a commie spy. When he was first accused, the old boy network of MI6 rallied around him and refused to admit his guilt. MI5, however, was always convinced of it. They could not prove anything, and Philby was more or less exonerated. Years later, they definitely had the proof, and Philby slipped off to the USSR.
There may be some who admire him because they are commies, but AFAIK he has never been championed as an "innocent" the way Hiss was here (along with the Rosenbergs).
Okay, I'll take your word. For what it's worth, guilt or innocence of the charges isn't what matters to leftists. If you doubt that, look the Hillary Clinton and her e-mails. What matters to lefties is if you are a true believer and if you walk the walk. If you do that they will make every possible excuse for you regardless of the evidence or, indeed, your own admissions. To them it's better for one guilty lefty to walk free than the cause take a setback. Indeed, lefties getting away with crimes is good because it encourages other lefties to commit crimes in the name of the cause with the assurance there will be no consequences. Consider the russians starving because of stupid dictatorial policies in the 1920s - 1930s but the NY Times pretending it wasn't happening because it would hurt the perception of communism as this great thing to strive for.
In the case of Philby, I didn't think lefties thought he was innocent per se, but rather that his sins weren't so bad as to warrant the punishment (well, the one he'd have gotten if he didn't defect). Because, you know, spying for the commies isn't a bad thing if you are a lefty. But if that was mistaken, as i said I'm not a brit so I'll accept that.