Did anyone in the UK ever call left wingers "The Radical Left" pre Trump. I know it's a description used in the US for decades but can't remember it being used in this country until a few years ago, AFAIR, in the past it was always the Extreme Left or Far Left.
Not an expert by any means, but I’ve heard the phrase a few times in books I’ve read.
Wilt (1976) - Tom Sharpe
Wilt classed himself with the Indifferents. In earlier years he had belonged to the Left politically and to the Right culturally. In other words he had banned the bomb, supported abortion and the abolition of private education and had been against capital punishment, thus earning himself something of a reputation as a radical while at the same time advocating a return to the craft of the wheelwright, the blacksmith and the handloom weaver which had done much to undermine the efforts of the Technical staff to instil in their students an appreciation of the opportunities provided by modern technology.
The Wilt Alternative (1979)
But Bilger was already out of the office and Wilt was left with the problem of finding some plausible excuse to offer the Committee. Not that he would have minded getting rid of Bilger but the idiot had a wife and three children and certainly couldn’t expect help from his father, Rear-Admiral Bilger. It was typical of that kind of intellectual radical buffoon that he came from what was known as ‘a good family’.
Also seen several references over the years. For instance, Citizen Smith;
The 1970s were certainly a time of seemingly never-ending industrial disputes, heavy discontent and disenchantment with the tiring older generation of British post-war politicians. Citizen Smith is a product of that period and echoes the 1970s ground-swell of frustration with politics[5]. Citizen Smith is far from the extremely sophisticated and elaborated anti-establishment cynicism and surrealism of Monty Python’s Flying Circus (1969-1974) but certainly draw on the same well-established British tradition of parody, sarcasm and irony that gave the British public permission to laugh at others and at themselves regarding British history and politics (Blackadder, Yes Minister), class divisions (Butterflies), WWII (Dad’s Army) and, last but not least, revolution, radical left and urban guerrilla (Citizen Smith).
So 'radical left' has been around since at least the 70s and featured in several fiction books I'd read around the time.