Some people are unable to recognize great architecture when they see it. Saarinen's building is applauded as one of the finest modernist structures in Britain and is rightly Grade II listed.
Some people are unable to recognize awful architecture when they see it. I like a lot of Saarinen's work, especially his furniture, but even Homer nods, and his Grosvenor Square building looks like a cinderblock with a grudge.
The vocabulary he chose to express this building is rather ungraceful in concrete, quite unlike his curvilinear works. The main issue is that the post-beam elements in the facade are poorly scaled. I haven't seen the plans, nor the section, so possibly the 'masterpiece' is inner beaty, but on the outside, it is a jumble of poorly scaled elements.
Also to call this, as the (know-it-all) Economist does, a "modern classic" is wrong. This is a pseudo-brutalist work, hardly high modernism. The contemporary Seagram's building (1958) is Modernism.
Another contemporary work relevant here (Paul Rudolph's Yale School of Architecture, 1963) shares some of the vocabulary of this work. And Rudolph knew how to design rectilinear concrete buildings. Saarinen clearly didn't.
[p.s. It is possible that the scale distortions in the front are (building) program driven, to address security considerations. The crit remains.]