On what basis? Born into wealthy family, worked as a barrister, decorated combat veteran, vociferous critic of appeasement, served in coalition government with Churchill, supported decolonisation, especially in Palestine, supported NATO formation, promoted US rebuilding of Europe, sent troops to Korea. Yes he implemented huge social and industrial reform but none of his policies were particularly problematic with the right and the workers all benefited greatly from nationalisation.
You'd have to be extremely ignorant to call him a loony lefty compared to those who receive that label nowadays.